Robinson and Related Annelations

Robinson and Related Annelations.—Since Robinson described his celebrated annelation reaction some forty years ago, it has remained one of the most widely used methods for six-ring synthesis. There have been several variants and modifications of the first described procedure, and this year two excellent reviews of the reaction and its variants have appeared (see Review Section). 

In a sequence which actually represents an anomalous Robinson annelation. Konst et al. have shown that enamines of either (134) or (136) are alkylated by MVK at the most substituted a-positions, affording bicyclic enones(135)and(137). 

Reagents i. Mg ii, POCI3-C5H5N iii, B2H5-H2O2 iv, Collins oxid., v, HCIO4 vi, KOH 

Reagents i, NaNH2-NH3-Et20 ii, HO(CH2)20H-TsOH-C6H6 iii, Bu Li-MeI-THF iv, HCI-H2O V, HgS04-Me0H vi, MeONa-MeOH 

It appears that 1,5-diketones which normally fail to form under standard basic Michael addition conditions may now be accessible by conducting the reaction in an acidic medium.The technique has been successfully applied to the 1,5-diketone (93). 

Weiler et al have employed their jS-keto-ester dianion y-alkylation technique to produce, in one step, 5-alkoxy-j8-keto-esters of known utility in Robinson-type annelation sequences.  

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Robinson and Related Annelations.—Methodology has been developed for the use of the more readily available l,3-dichlorobut-2-ene (91) in place of the dibromo-homologue previously employed as a pentenone equivalent in the Robinson annela-tion sequence (Scheme 23). The proximity of the carbonyl group in the methyl acetylene intermediate (92) aids the stereospecific hydration of the acetylenic linkage, leading to the necessary l,S-diketone intermediate. 

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