Carbocycle synthesis

Medium sized carbocycle synthesis Ring contracting [1,2]-Wittig rearrangement Yadav and Ravishankar have demonstrated that the Wittig rearrangement of the cyclic substrate 32 is useful for the construction of the taxane skeleton 33, albeit in low yield (equation 17). ... 

The reaction of enamines and imines with acrylamide results in aza-annulation120,121. Other electrophilic alkenes which have been used to alkylate enamines and the products used in hetero- or carbocyclic synthesis include ethyl / -nitroacrylate122, where reaction occurs beta to the nitro not the ester group, 2-(phenylseleno)prop-2-enenitrile [CH2 = C(SePh)CN]123,124, phenyl a-phenylselenovinyl sulphone [CH2 = C(SePh)-S02Ph]124 and phenyl a-bromovinyl sulphone124. An electrophilic allene, phenylsulpho-nylpropadiene, has also been used to alkylate enamines125 (Scheme 44). 

A similar [4 + 2] cycloaddition across the a, -positions has been observed in the reaction of isophorone dienamines with electrophilic alkenes (vide infra), and a similar ring opening has been postulated in the reaction of methyl vinyl ketone with dienamines in aprotic media (Section VII.C). Confirmatory evidence for the role of the C-3 methyl group in the above reaction was obtained from reaction of 8-methyl-A 1,8a-2-octalone dienamine with methyl acrylate only the /Talkylated product was obtained in protic and aprotic solvents14. The reaction of dienamines with electrophilic alkenes is discussed further in the section on carbocyclic synthesis (Section VII.C). 

Pinaeol cycUzation. A key step in a new method for carbocyclic synthesis applicable to gibbcrcilic acids involves the pinaeol cyclization of (I) to (2). This was accomplished, in 75% yield, by addition of (I) to a mixture of magnesium amalgam (7.5 eq.) and dimcthyidichlorosilane (2 eq.) in dry THF at 25° followed by subsequent alkaline... 

A detailed survey of total syntheses of heterocyclic steroidal systems, belonging mainly to the oestratriene series, has appeared recently. A first approach (Scheme 13), based on the Torgov carbocyclic synthesis, " uses appropriate... 

Alonso, R A, Burgey, C S, Rao, B V, Vite, G D, Vollerthun, R, Zottola, M A, Fraser-Reid, B, Carbohydrates to carbocycles — synthesis of the densely functionalized carbocyclic core of tetrodotoxin by radical cyclization of an anhydro sugar precursor, J. Am. Chem. Soc., 115, 6666-6672, 1993. 

The regiospecific 5-exo-trig cyclizations of substituted 5-hexenyllithiums was exploited for the stereoselective synthesis of bicyclic systems by tandem cyclization of acyclic diolefinic alkyllithiums [42]. However, this carbocycle synthesis has three major limitations ... 

Ichikawa, J. Wada, Y Fujiwara, M. Sakoda, K. The nucleophilic 5-endo-trig cyclization of 1,1-difluoro-l-alkenes ring-fluorinated hetero- and carbocycle synthesis and remarkable effect of the vinylic fluorines on the disfavored process. Synthesis 2002,13, 1917-1936. 

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