Cyathin diterpenes

In a study aimed at synthesizing the cyathin diterpene skeleton, another pyrylium ylide-alkene [5+2] cycloaddition was employed (as shown in Scheme 13) <1999T3553>. 

The intermolecular [2 + 2]-photocycloaddition of para-tetrahydronaphthoqui-nones has been applied by Ward et al. to the synthesis of cyathin diterpenes [52], An example is represented by the total synthesis of ( )-allocyathin B3 (46), during the course of which the diastereoselective [2 + 2]-photocycloaddition of allene to substrate 44 served as one of the pivotal steps (Scheme 6.17) [53]. The addition delivered a mixture of regioisomers (r.r. = 80/20), from which compound 45 was separated. The facial diastereoselectivity was perfect due to the concave shape of the quinone. 

Ward, D.E., Gai, Y., Qiao, Q., and Shen, J. (2004) Synthetic studies on cyathin diterpenes - Total synthesis of ( )-allocyathin B3. Canadian Journal of Chemistry, 82, 254-267. 

D.E. Ward et al. reported a general approach to cyathin diterpenes and the total synthesis of allocyathin 63. The tetracyclic secondary alcohol was converted to the corresponding ketone using TPAP/NMO in good yield. ... 

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