2.24 – Pyrans and Fused Pyrans: (iii) Synthesis and Applications

Pyrans and their Benzo Derivatives - Synthesis

47 – Compounds of Thallium in Organic Synthesis

2.23 – Pyrans and Fused Pyrans: (ii) Reactivity

Only thallium and indium of the group IIIB elements are known to exist as the monovalent ions under normal conditions. By contrast with indium(I) salts, however, which are very readily oxidized to the trivalent state and the chemistry of which is not at all well developed, thallium(I) salts are stable. Indeed, it has been accurately stated that “… for thallium the TlI–TlIII relationship is a dominant feature of the chemistry.”1 This is certainly true with respect to the utility of Tl(III) as an oxidant in organic chemistry. The reduction potential E o for Tl3+ + 2e → Tl+ in aqueous solution under standard conditions is + 1.25 V, 2 but clearly more powerfully oxidizing Tl(III) species can be obtained by variation in the anion associated with the metal and by appropriate choice of reaction conditions. In the terms of the organic chemists, the thermodynamically favorable reduction of Tl(III) to T1(I) is one of the most important “driving forces” in thallium(III)-mediated oxidations.

Thallium(III) Salts as Oxidants in Organic Synthesis

Photosensitized oxygenation of 5,6-dihydro-4-methyl-2H-pyran (1) in methanol followed by reduction gave two kinds of allyl alcohols [(2) and (3)] together with the epoxy-ethers (4) and (5). When the reaction was carried out in solvents other than alcohols it gave (2) and (3) exclusively. Ozonization of 4-methylenepyrans (2) and (13) followed by reduction with zinc powder–acetic acid afforded 3-hydroxytetrahydropyrans (11) and (14). Bromination of (14) gave the dibromide (16), which yielded bromopyran (17) and 3-acetoxy-4H-pyran-4-one (18) on treatment with 1,8-diazabicyclo[5.4.0]undecene. However, the reaction of (16) with silver acetate in acetic acid predominantly afforded (17) in excellent yield. Other allied reactions and mechanisms of these reactions are described.

Hiroyuki Adachi

Papers

Synthesis and reaction of 3-hydroxytetrahydro-4H-pyran-4-one