Showing papers on "Pyran published in 1993"

Total synthesis of hemibrevetoxin B and (7a.alpha.)-epi-hemibrevetoxin B

Abstract: The total synthesis of hemibrevetoxin B (1) and (7aα)-epi-hemibrevetoxin B (2) is described. The synthesis of the epimer (2) was achieved through a convergent approach involving coupling of the carboxylic acid 17 carrying the bicyclic pyran system with the hydroxy compound 31 containing the monocyclic pyran system, thionation of the resulting diester 32 to the dithionoester 33, photolytic closure to the oxepane enol ether 34, and hydroxy ketone cyclization to the dioxepane system 40. The Z-diene system was established using a selenyl-Wittig reaction followed by syn elimination of the selenoxide to the diene. The α-vinyl functionality was installed using the Eschenmoser's salt methodology. The synthesis of hemibrevetoxin B (1) was achieved through a linear approach involving sequential formation of the oxepane rings (65 → 67 → 73) using the method of thionolactone formation followed by nucleophilic addition and regio/stereoselective hydroboration (67 → 68, 75 → 76). Elaboration of the side chains was carried out in a similar fashion as described for the epimer. The streochemistry of the ring junctures in 1 and 2 and intermediates leading to them was established by X-ray cristallographic analysis carried out on compounds 45 and 54

Comparative photodegradation study between spiro[indoline—oxazine] and spiro[indoline—pyran] derivatives in solution

Abstract: The photochromic compounds 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3′-[3H]naphth[2,1-b][1,4]oxazine], 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,2′-[3H]naphth[1,2-b]pyran] and 1,3-dihydro-8′-methoxy-6′-nitro-1,3,3-tri- methylspiro[2H-indole-2,2′[3H]benzopyran] were degraded under UV light irradiation in toluene solution. The resulting main photoproducts separated by gas chromatography and high performance liquid chromatography were identified by different couplings with UV—visible diode array detection, mass spectrometry and Fourier transform IR spectroscopy and were compared with synthetic standards. The different nature of the photoproducts involving the chromene part of the molecules could suggest different mechanisms of degradation between the spiropyran and spiro-oxazine series and could explain the better fatigue resistance of the latter.

Heterogeneous photocatalysis. XI: Zinc sulphide catalysed dehydrodimerization of dihydropyrans and cyclohexene

Abstract: An aqueous suspension of zinc sulphide photocatalyses the dehydrodimerization of 3,4-dihydro-2 H -pyran, 4-methyl-5,6-dihydro-2 H -pyran and cyclohexene under concomitant hydrogen evolution Maximum turn-over numbers are in the range of 3000 monolayers While three regioisomers are obtained from the two first substrates, cyclohexene affords the 2,2′ isomer exclusively The most reactive zinc sulphide samples have specific surface areas of about 100 m 2 g −1 and an excess of sulphur at the surface These samples do not suffer photocorrosion when irradiated in a suspension of pure water However, this occurs with samples of lower surface area and excess zinc at the surface

Fluorine-19 NMR studies of the mechanism of riboflavin synthase. Synthesis of 6-(trifluoromethyl)-8-(D-ribityl)lumazine and derivatives

Abstract: 6-(Trifluoromethyl)-8-(D-ribityl)lumazine (17) was synthesized in order to study its reactivity at C-7 and its binding to riboflavin synthase of Bacillus subtilis. Compound 17 was prepared by reaction of 5-amino-4-[(D-ribityl)amino]-2,4-(1H,3H)-pyrimidinedione hydrochloride (3-HCl) with trifluoropyruvaldehyde hydrate (18). NMR studies revealed that under basic conditions, 17 forms only one major anionic species in which the oxygen of the 3'-hydroxyl group on the ribityl side chain binds covalently to C-7 of the lumazine, resulting in the formation of a pyran ring

Structure and synthesis of arnottin I : a 6H-benzo[d]naphtho[1,2-b]pyran-6-one derivative from a plant source

Abstract: Arnottin I 2 isolated from Xanthoxylum arnottianum Maxim and of previously unknown structure has been unambiguously synthesized utilizing a 2-methylarenofuran as a masked salicylaldehyde This is the first occurrence of a 6H-benzo[d]naphtho[1, 2-b]pyran-6-one derivative, the same skeleton as that of gilvocarsin-type antibiotics showing antitumour activity, from a plant source

A new route to trienals using 2-substituted 2H-pyran-based Wittig reagents

Abstract: A procedure is presented which allows the six-carbon homologation of aldehydes to give the corresponding trienals viathe likely intermediacy of the novel 2-triphenylphosphranylidenemethyl-2H-pyran; application of this methodology to a short synthesis of the marine alarm pheromone, navenone B, is also described.

Stereocontrolled Syntheses of Polysubstituted 2,3‐Dihydro‐4H‐pyran‐4‐ones by cyclocondensation of β‐ acyloxy‐ketones

Abstract: syn-β-Acyloxy-ketones 6 and an anti-β-acyloxy-ketone 17 undergo smooth intramolecular enolate/ester condensations 6 [RIGHTWARDS ARROW] 18 and 17 [RIGHTWARDS ARROW] 19 when treated with TiCl4/EtN(i-Pr)2. Thus, tri- and tetrasubstituted cis- or trans-2,3-dihydro-4H-pyran-4-ones are easily prepared in a stereoselective manner.

Photoinduced molecular transformations. Part 137. New general synthesis of 3-substituted 3,4-dihydro-1H-benzo[2]pyran-1-ones (3,4-dihydroisocoumarins)via radical and photochemical fragmentations as the key step

Abstract: A new general method for a two-step synthesis of 3-substituted 3,4-dihydro-1H-benzo[2]pyran-1-ones (3,4-dihydroisocoumarins) is described. This method involves either regioselective alkoxyl radical fragmentation or regioselective photochemical fragmentation as the key step. 2-Hydroxyalkylation of a lithiated o-tolyl tert-butyl ketone with aromatic and aliphatic aldehydes and ketones gave equilibrated mixtures of 1-tert-butyl-3,4-dihydro-l -hydroxy-3-alkyl (or 3-aryl)-1H-2-benzopyrans and their ring-opened isomers in 42–94% yield. Photolysis of mixtures in chloroform with Pyrexfiltered light gave 3-alkyl (or 3-aryl)-3,4-dihydro-1H-benzo[2]pyran-1-ones (3,4-dihydroisocoumarins) in 27–64% yield as exclusive isolable products. On the other hand, photolysis of hypoiodites of the equilibrated mixture in benzene containing mercury(II) oxide and iodine gave the 3-substituted dihydroisocoumarins in 37–64% yield with an accompanying formation of phthalide, arising from a radical cascade process triggered by β-scission of the alkoxyl radicals generated from the ringopened isomer of the lactones. The formation mechanisms of all the products are discussed.

Use of functionalised ynamines in a hetero-Diels–Alder approach to dihydronaphtho[1,2-b]pyrans and indeno[1,2-b]pyrans

Abstract: Reaction of the ynamine ester methyl 3-(pyrrolidin-1-yl)prop-2-ynoate 5 with 2-(4-nitrobenzylidene)1-tetralone 1 results in a very poor yield of the chromatographically labile 4-aryl-5, 6-dihydro-4H-naphtho[1,2-b]pyran 8 along with the α-pyrone 10. Increasing the reactivity of the 4π component by using the 2-arylidene indan-1,3-diones 11–13 results in moderate to good yields of the 4-aryl-5-oxo-4H-indeno[1,2-b]pyran-3-carboxylates 14–19. An ynamine nitrile 24, generated in situ, also reacts with 12 and 13, furnishing rather lower yields of the adducts 20 and 21.

Reaction of Methyl 2-Bensoylamino-3-dimethylaminopropenoate with Heterocycilc Hydroxy Compounds. The Synthesis of Fused Pyranoazines

Abstract: Methyl 2-benzoylamino-3-dimethylaminopropenoate (1) reacts in acetic acid with monocyclic or bicyclic heterocyclic compounds with two hydroxy or potential hydroxy groups in 1,3-position to give fused pyranones. Accordingly, derivatives of 2H-pyrano[3,2-c]pyridine (3), 2H-pyrano[3,2-c]quinoline (9, 10,and 11), 2H-pyrano[2,3-d]pyridazine (13), 8H-pyrano[3,2-d]tetrazolo[1,5-b]pyridazine (16), pyrano[4,3-b]pyran (18), and 2H-pyrano[2,3-c]-pyridine (22) were obtained

Structure of a chalcone

Abstract: 1-(7-Hydroxy-2,2-dimethylchroman-6-yl)-3-phenyl-2-propen-1-one, C 20 H 20 O 3 , M r =308.38, P2 1 /a, a=12.002 (2), b=11.013 (2), c=12.652 (3) A, β=106.24 (3) o , V=1605.6 A 3 , Z=4, D m =1.26 (2), D x =1.275 Mg m -3 , λ(Mo Kα)=0.71069 A, μ=0.079 mm -1 , F(000)=656, T=293 K, R=0.037, wR=0.034 for 892 reflections with I>3σ(I). There is an internal hydrogen bond O-H...O between the carbonyl O atom in the cinnamoyl group and the hydroxy group of the chroman moiety. The conformational analysis shows that the phenyl ring is trans with respect to the double bond linking it with the chroman group. The pyran ring is in the prevalent half-chair conformation

1H Nmr spectra of prenylated flavonoids and pyranoflavonoids

Abstract: 1 H Nmr examination of prenylated flavonoids has shown that the presence of the prenyl group at C-3, C-6, or C-8 position of flavone can be deduced from the chemical shift of methylene protons of prenyl group measured in acetone-d 6 . The chemical shifts of olefinic proton on pyran ring and A ring proton can be used to distinguish between linear type pyranoflavone and angular type pyranoflavone. The application of this 1 H nmr method to the identification of Atalantia flavone and Artocarpus flavones is discussed

Reactions of dianions of acyclic β-enamino ketones with electrophiles. Part 5. Esters: synthesis of pyridin-4-one and pyran-4-one derivatives

Abstract: The electrophilic attack of esters to dianions of β-monosubstituted amino-α,β-unsaturated ketones and the subsequent cyclisation of the addition product offers a valuable generalisation of the synthesis of N-substituted pyridin-4-ones and pyran-4-ones. The reaction proceeds in good to high yields with α′-dianions. A side metallation reaction is observed with aliphatic esters. Product distribution is influenced by the nucleophilic ability of nitrogen and the electrophilicity of the carbonyl group which are involved in cyclisation. The reaction of γ-dianions with esters is the first example of a reactivity of these systems which resembles alkyl organometallic reagents more than enolates. In fact the major product of this reaction is the alcohol with incorporation of two enaminone moieties.

Cyanoacetic acid hydrazide in heterocyclic synthesis: A new route for the synthesis of several annelated pyran derivatives

Abstract: Cyanoacetic acid hydrazide reacted with some 2-pyrazolin-5-one, isoxazol-5-one and 2-thiazolin-4-one and their ylidene derivatives to yield several new annelated pyran heterocycles. Structures were established on the basis of elementary analyses and spectral data studies in addition to synthesis via other routes.

α-Pyrones. Part 4. Synthesis of 3-benzoylamino-6-(indol-2-yl)pyran-2-ones and their rearrangement to substituted azepino[1,2-a]indole-6-ones: unusual neighbouring group participation

Abstract: 6-(2-Nitrostyryl)pyran-2-ones 3 are converted into 6-(indol-2-yl)pyran-2-ones 4 by reduction of the nitro group with triethyl phosphite and consequent addition of the nitrene intermediate to the styryl double bond. Compounds 4 undergo a rather unusual, base-catalysed, rearrangement leading to azepino[1,2-a] indol-6-ones 5.