Vitamin diol dehydration

Fig. 21. Scheme of Silverman and Dolphin for vitamin B12-catalyzed diol dehydration (87). 

By the Darzens reaction. -ionone afforded a C14 aldehyde which as the diethyl acetal underwent addition to ethyl vinyl ether in the presence of boron trifluoride etherate to yield a Cl6 acetal. After hydrolysis, loss of ethanol and formation of the diethyl acetal as before, reaction under acidic condiions with ethyl propenyl ether gave the unsaturated Cl9 aldehyde after hydrolysis and removal of ethanol. Reaction of two moles with ethyne dimagnesiuro bromide produced the C40 chain and dehydration of the diol, selective catalytic hydrogenation followed by isomerisation completed a remarkable technical synthesis of i-carotene. Further variations have involved the use of two moles of the C14 aldehyde and a Cl2 divinyl ether. An independent approach (ref.29) has utilised vitamin A (32) converted to a phosphonium salt, thence to the corresponding phosphoran, autoxidation of which afforded s-carotene ( scheme 16). 

In addition to P-carotene, freshwater fish can also convert the xanthophyl lutein (5-12), also known as 3,3 -dihydroxy-a-carotene or (3R,3 S,6 R)-p,8-carotene-3,3 -diol, into vitamin A. Lutein eliminates one molecule of water (C-3 hydroxyl group) and yields anhydrolutein (5-13), which cleaves to the corresponding aldehydes. These aldehydes are reduced to a l-trans-3,4-didehydroretinol (vitamin A2) and aU-trans-3-hydroxyretinol, respectively. The latter compound can dehydrate to form another... 

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