Terpenes retinals 334

A number of compounds important to animal physiology have been identified as isoprenoid compounds. Notable examples are vitamin A, retinal (Section 28-7), and squalene (Table 30-1). Also, terpene hydrocarbons and oxygenated terpenes have been isolated from insects and, like famesol, show hormonal and pheromonal activity. As one example, the juvenile hormone isolated from Cecropia silk moths has the structure shown in 3 ... 

Vitamin A terpenes.2 A novel entry to retinal (5) involves reaction of the chloride (2) with 1 to give an intermediate (3) in 55% yield. Oxidation of 4 with DMSO in the presence of N,N-dicyclohexylcarbodiimide gives rise to the four 11,13-stereoisomeric retinals (5). Addition of I2 increases the proportion of the all-/rara-retinal (E,E-5). 

For syntheses of vitamin A aldehyde (retinal) and higher terpene aldehydes, appropriate 2-methyl-2-butene-1,4-dials (32) with a protected aldehyde function were used as the C5 building blocks. They are accessible, in turn, via the C5 building block p-formylcrotyl acetate (8 b) in the form of its dimethyl acetal, or via P-formylcrotyl methyl ether (29)34 b) ... 

The effective electroreductive pinacolization has been exemplified in the terpene series, i.e. aldehydes such as retinal and ketones such as a- or j8-ionone give the corresponding pinacols in good yields when electrolyzed in a DMF-sodium per-chlorate-Hg system or with a Pt cathode in the presence of tin [8, 9]. 

Formulas of terpenes in the scheme above are presented in two rows. In the bottom row, if the chemical bonds labeled in red are deleted the residues represent isoprene units. Based on the number of isoprene fragments, menthol is monoter-pene, a-selinene belongs to sesquiterpenes and cembrene as well as vitamin A are diterpenes. Menthol is the component of peppermint responsible for its refreshing odor. The aroma of celery originates from a-selinene while cembrene can be isolated from pine trees. The derivative of retinal has already been discussed in the section about the cis-trans isomerism in the biochemical process of vision. 

Notice (from your answer to Problem 15.8) that all of the isoprene units in farnesol and retinal are joined in a head-to-tail manner, but that they are not all arranged that way in squalene and jS-carotene. Where does the head-to-tail arrangement break down in these two terpenes Hint Count carbons to locate the terpene units.) What does this suggest about the way these two terpenes might be bio synthesized ... 

As condensation with a, 8-unsaturated compounds occurs exclusively in a 1,2-fashion, this reagent has been used in terpene synthesis. A synthesis of retinal (3b) in three steps from 8-ionone (2a) (via /3-ionylideneacetaldehyde and Cig ketone) was reported using reagent 1 for the first and third steps (eq 2). ... 

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