Robinson-type cyclization

Cyclization product 6 was selectively obtained from the racemic precursor 5 as a result of the kinetic resolution in Robinson-type cyclization (Scheme 3.3). ... 

Syntheses of diastereomerically pure racemates of himachalene derivatives started from cycloheptanone G (Fig. 9). The sequence to I involved dimethyla-tion to yield H followed by bromination/dehydrobromination and conjugate methylation using cuprate chemistry. The sequence furnishing L and M follows a Robinson-annelation type Reaction of I with 3-(trimethylsilyl)but-3-en-2-one yielded K. Refluxing K with potassium hydroxide in ethanol removed the silyl group and cyclized the diketone to form a 97 3 mixture of racemic L and M. Occurring as a volatile in A.flava, L served as a versatile intermediate in the syntheses of other Aphthona compounds. 

The total synthesis was achieved in 1954 by Woodward and his collaborators (156). Before this, various unsuccessful attempts had been made to make a start toward a synthesis of strychnine (179, 180, 181, 182,) but these are now of little interest, with the exception of Robinson s idea (182) to emulate the postulated biosynthesis by attempting to synthesize the dialdehyde CCXVI, which then might be induced to cyclize by a combination of Mannich and aldol type condensations to the Wieland-Gumlich aldehyde (LV) the synthesis of CCXVI unfortunately was not realized. Much more recently, however, this idea has been used by van Tamelen et al. (184), who successfully synthesized the dialdehyde CCXVII and converted it in aqueous acetic acid-sodium... 

Intramolecular aldol condensations are often more successful than the inter-molecular type. A particularly important example of the synthetic use of intramolecular condensation is in the Robinson annelation, a procedure that constructs a new six-membered ring from a ketone that has an enolizable hydrogen. The stages in which this alkylation-cyclization occurs are outlined below. The cyclization... 

The second group of syntheses effected via ABD derivatives linked by a Cg—Ci4 bond was first proposed, also by Robinson, in 1938 [374]. In these syntheses, ring C is formed by the cyclization of acids of the type of (270) (Scheme 26) obtained, in their turn, from the y-diketo acids (268) with an aromatic substituent comprising rings A and B and a side chain comprising atoms Cij—C17. This method leads directly only to equilenin derivatives, but the reduction of these has enabled some derivatives of estrone to be obtained. 

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