Synthons bipolar

A new aspect of synthon application in the use of acetylene (and, in general, alkynes) emerged with the development of methods for the carbometallation of triple bonds in accordance with the Scheme 2.96. As can be seen from the scheme, the net contribution of acetylene in this sequence of conversions corresponds to the insertion of a bipolar C2 unit (synthon 233 with a Z-coniiguration of the double bond) between the nucleophilic and electrophilic fragments. 

The universal applicability of acetylene as this type of bipolar synthon and its efficiency in the synthesis of natural compounds has already been demonstrated (Section 2.3.3.). 

Methods developed to carry out this reaction under aprotic conditions with an interception of the intermediate carbanion by an external electrophile, however, allowed this same reagent to be used as an equivalent to the bipolar CH2 CHC0CH3 synthon, as is represented in the following general scheme ... 

Transition metal chemistry provides an especially rich source of methods for the construction of cyclic compounds. The idea to synthesize 297 from precursor 298 comes from the recognition of 298 as an equivalent to the 1,3-bipolar synthon 298a, trimethylenemethane. It was reasoned that intermediates equivalent to 298a might be formed as transient species and stabilized by complexation with transition metals, e.g. Pd(0). With reagents such as Michael acceptors they can be used for an efficient cyclopentanoannulation. ... 

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