Myers-Saito cyclization compounds

Since the double bond between C5 and C6 of the enynecumulene 169 is not required for the Myers-Saito cyclization, a large number of enyneallenes have been synthesized as model compounds for the neocarzinostatin chromophore and tested for DNA-cleaving activity in recent years, with the results having already been summarized extensively (cf. Chapter 20) [162]. 

The enyne-allene/cumulene transformation is not restricted to the Myers-Saito cyclization. The general analysis showed that the course of the cyclization of allene 3.968a (Scheme 3.146, where A = B = carbon) is determined by substitution in the acetylene branch and depends on electronic and steric factors, and the effect of stress in the cycle [424]. According to the study of the kinetic isotope effect, the formation of structure 3.970a can occur if there is at least one phenyl substituent in position 1 or 7. Depending on the substitution in the allene branch, there are several possible routes for the intramolecular reaction (formal [4+2] or [2+2] cycloaddition, or formation of ene-compound [424], which makes it possible to design a variety of ring systems. 

For the case of oxyanion, two factors were found to play a role. The size and nature of the ring in which enyne-allene is embedded as well as the steric bulk of the substituents of the allene and alkyne affects the competition between the two cyclizations. The Myers-Saito product is not observed for alkynes with the trimethylsilyl substituent even for cyclohexane-annulated compounds. However, when the alkynyl substituent is changed to phenyl, the resulting relief of steric strain in the C2-C7 transition state permits the cyclization of 25 to occur at low temperature and yields the styrene derivative 26 The cyclopentane-annulated compounds give the Myers-Saito product, or fail to react when a bulky silyl substituent is present at the alkynyl terminus. In contrary, the benzene-annulated compounds undergo rapid the Schmittel cyclization dominantly or exclusively. The cyclizations of oxyanion-substituted enyne-allenes studied in the cited article occured at far lower temperatures than the analogous cyclizations of neutral enyne-allenes. The presence of the oxyanion presumably stabilizes, by resonance, the diradical in the transition state. ... 

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