Carotenoids and related compounds. Part 38. Synthesis of (3RS,3′RS)-alloxanthin and other acetylenes

Abstract

The all-trans, 9-cis, and 9,9′-di-cis forms of (3RS,3′RS)-alloxanthin have been synthesised, also the 9-cis-isomer of (3RS,6′RS)-crocoxanthin. Perhydrogenation of alloxanthin gives a product with the same optical properties as perhydrozeaxanthin. These results confirm the formulation of alloxanthin as (3R,3′R)-7,8,7′,8′-tetradehydro-β,β-carotene-3,3′-diol, of diatoxanthin as (3R,3′R)-7,8-didehydro-β,β-carotene-3,3′-diol, of crocoxanthin as an enantiomer (probably 3R) of (6′R)-7,8-didehydro-β,ε-caroten-3-ol, of monadoxanthin as an all-trans-isomer of 7,8-didehydro-β,ε-carotene-3,3′-diol, and of pectenolone as an all-trans-isomer of 3,3′-dihydroxy -7′,8′-didehydro-β,β-caroten-4-one. Carotenoid isomers with the cis-configuration about the acyclic double bond adjacent to an acetylenic linkage in either the 7- or 7′-position were shown to be thermodynamically more stable than the all-trans-forms.