Summery:
A convenient method for the synthesis of [3]-cumulenes based on a Uorner-Emmons type reaction of allenyldiphenylphosphine oxides with aldehydes or ketones has been describ'ed. A facile two-step synthesis of eneynecumulenes has been accomplished by this method. Neocarzinostatin chromophore (NCS-Chr) is a new class of antitumor antibiotics with an unusual cycloalkendiyne structure and a remarkably high DNA-cleaving activity.' The mechanism of its DNA-cleaving process proposed by Myers et al.,2 involves the generation of a biradical species through a Bergman type cycloaromatization of the eneynecumulene 1. In connection with our studies on the action mechanism of NCS,s we disclosed that non-strained acyclic eneyneallenes undergo extremely facile cycloaromatization to generate a biradical species which causes cleavage of double stranded DNA under mild conditions.415 In an extension of these studies, we became interested in the biradical forming process of more closely related system, eneynecumulenes. NCSChr 1 While existing synthetic methods for [3]-cumulenes (1,2,3-butatrienes) include i)l,4elimination of various types of substituted acetylenes, 6 ii) coupling of metal-methylenecarbenoids7
vinyl-copper derivatives6 or vinyl-borane precursor@ and iii) a Wittig phosphacumulene strategy,10 vast majority of the previous methods are not useful for the synthesis of unstable, highly conjugated and partially substituted eneynecumulenes. A more convenient and versatile method for the construction of [3]-cumulene structure under milder conditions is apparently necessary. We now wish to report a convenient and general method for the synthesis of [3]-cumulenes based on a Horner-Emmons type reaction using allenyldiphenylphosphine oxides11 in combination with aldehydes or ketones which can be successfully utilized for the synthesis of a variety of eneynecumulenes.