By Armin de Meijere, Tse-Lok Ho, Henning Hopf, Rafael R. Kostikov, Isao Kuwajima, A.P. Molchanov, Eiichi Nakamura

Contents: I. Kuwajima, E. Nakamura, Tokyo, Japan: Metal Homoenolates from Siloxycyclopropanes.- R.R. Kostikov, A.P. Molchanov, Leningrad, USSR, H. Hopf, Braunschweig, FRG: Gem-Dihalocyclopropanes in natural Synthesis.- T.-L. Ho, Mt. Prospect, IL: Through-Bond Modulation of response facilities byRemote Substituents.

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These products in 57 Rafael R. Kostikov et al. turn are quite useful for further transformations to building blocks for organic synthesis [148]. R31 OphMe [ only iX=Ph R2~C 1 R1 R k'/- CI CI Lx Dehydrochlorination of pentachlorocyclopropane, formed from trichloroethylene and sodium trichloroacetate as a source of dichlorocarbene, yields tetrachlorocyclopropene [150], a particularly versatile reagent for various applications. It is a reasonably reactive dienophile [151], a reagent applicable to heterocyclic synthesis [152], and an electrophile for aromatic substitutions [153] and additions to alkenes [154] in the presence of Lewis acids.

The trichlorotitanium species thus obtained have been shown (XH NMR) to be similar to that generated along the siloxycyclopropane route and indeed exhibit very similar reactivities. This method does provide a conventient alternative to the siloxycyclopropane route. O HH Bu3Sn/-',v,COX. TiCI4 Ci3Tr~ X RCHOR,J~OX (48) 50-85"/, X = OR, NR2 Isotopic labelling, Eq. (49), indicated that the transmetalation occurs via direct tin-carbon bond cleavage rather than via intermediate formation of an alkoxycyclopropane [45].

Eq. 67), however, indicates that they are not identical. 1 ~ /OSiR33 1 J#OSiR33 24 ~1 '+" 3"~/,~OSiRs 25 (67) 33 Isao Kuwajimaand Eiichi Nakamura Scheme 5 summarizes the regiochemistry of ring opening of methyl- (6) and phenyl-substituted alkoxy-siloxy-cyclopropane (7) by LiOMe, ZnC12, HgCI2, and TIC14, as well as the 13C NMR chemical shifts of the respective metal methyls [27]. The NMR data correlate with the nature of the metal-carbon bond: The more polarized it is, the less positive the chemical shift of the methyl group.

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