Alkylated and dealkylated retinals

Methyl groups at positions 5, 9 and 13 on retinal are not a necessity for pigment formation. This was shown by synthesis and formation of visual pigment analogs from 5-desmethyl, 17 [145], 9-desmethyl, 18 [70,176-180], 13-desmethyl, 19 [176-180] and 9,13-desmethylretinal, 20 [70,176,177], 

Changing the position of the methyl group on the side chain, such as in 13-desmethyl-14-methylretinal, 21 [162], or the increase of the bulk of the side chain by additional methyl substitution such as in 10-methyl, 22 (K. Nakanishi, unpublished results), 14-methyl, 23 [162] and 10,14-dimethylretinal, 24 (Nakanishi, unpubl.  

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