Vinylcyclopropane heteroatom

The heteroatom version of the vinylcyclopropane rearrangement serves to facilitate alkaloid construction. Scheme 13 outlines a strategy for the pyrrolizidine alkaloid isoretronecanol 211 90). Use of a carboxaldehyde (i.e. 213) as a synthon for the primary alcohol provides an ability to adjust stereochemistry. It also sets up formation of the pyrrolidine ring bearing the aldehyde by an aldol-type condensation of an enol of the aldehyde onto an imine derived from 214. Because of the lability of such systems, introduction of X=PhS imparts stability. The resultant azacyclopentene translates to an imine 215 using the iminocyclopropane rearrangement methodology. Simple condensation of the primary amine 216 with aldehyde 37a then initiates this... 

A summary of the synthetic methods based on the vinylcyclopropane rearrangements s pears in Section 8.1.8. The listing of total syntheses featuring this rearrangement and its heteroatom variants is compiled in Section 8.1.9. 

Heteroatoms, depending upon their location, have varied effects. Thus, for instance, the pyrolysis of l,l-dichloro-2-methyl-2-vinylcyclopropane (180) at 200-275 C yielded exclusively l-methyl-4,4-dichlorocyclopentene (181) whereas its isomer, l,l-dichloro-2-(l-methylcyclopropyl)ethylene (182) was remarkably stable up to 400 °C (equation 123) . 

Synthesis of 1-Hetero-l-vinylcyclopropanes by Elimination of Two Heteroatomic Moieties... 

The most fascinating features of vinylcyclopropane reveal themselves upon the introduction of not only substituents but heteroatoms. The number of possible permutations of molecular connectivity in a system such as (10 Figure 1) is virtually limitless. (Consider the series of connectivity operations for a... 

Although simple vinylcyclopropanes have not proven useful, Fowler reported in 1971 that a homopyrrole, a constrained and heteroatom-activated vinylcyclopropane, reacts with alkynes to produce a seven-membered ring product (Eq. 44). A zwitterionic intermedi-... 

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