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!ii:?~o:b~~' sho thaata of the hindered 7&s and 7,9dcb isomers of 3-dehydraethd from , Xi~rocedunz, involving introdtion of the hinder& 7-d geomehy prior to aldel&da. he appltcable to preparatton of other unknown isomer of Vitamin A2 The discovery of a photochemical method to introduce the hindered 7-ds geometry in derivatives of vitamin A* led to the synthesis of all sixteen possible isomers of retinal.2 The situation, however, has been different for the closely related 3-dehydroretinal, 1 (vitamin A2 aldehyde), which is also important in vision research because of its 114s isomer being the chromophore in porphyropsin pigments. 3 However, in recent years, only one new isomer has been added to the earlier known six isomera of vitamin A2 (all-trans, 13-&s, ll-ds, 9-c& 11,13dicia and 9,13_dicis)*: the 74s isomer obtained from direct irradiation of the trans isomer.5 The lack of progress toward prepa-ration of new isomers of this derivative of vitamin A is partly due to the altered photochemical properties of its lower homologs,6 making building blocks containing the hindered 7-cis geometry relatively inaccessible. In this paper we describe a rational synthetic design to 7-ds-3-dehydroretinal.
The method is potentially a general one, applicable to preparation of other unknown isomers of 34ehydroretinal.