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The isomerically pure Z-and E-pentenylboronates 2 and 3 addition of other Z-and E-pentenylmetal compounds to were added to ketones under 4 to 8 kbar pressure resulting in ketones. The diastereoselectivities obtained in the addition of the homoallyl alcohols 10 -13 with different diastereoselecthe configurationally undefined pentenyl Grignard (4) and tivities. These reactions were considered as models for the titanium reagents (5, 6) is discussed against this background.The diastereoselectivity of the addition of crotyl Grignard reagents (la) to ketones has not been investigated in detail'). However, good levels of anti-stereoselectivity were reported for the addition of crotyltitanium compounds (lb, 1~)~) or of crotyl-9BBN (ld)4) to ketones.

Intramolecular allylboration reactions of (8-0x0-2-octeny1)boronates proceed with excellent diastereoselectivity: the E-isomer I d leads to the trans-2-vinylcyclohexanol(2) with 99.5% ds., the corresponding Z-isomer 3d cyclizes to give the cisisomer with a diastereoselectivity exceeding 99.8%.Cyclization reactions leading to 6-membered rings are highly valued synthetic operations, especially if the new stereocenters can be set up with defined configuration in the cyclization step. The following transformations can be considered as prototypical: -1 2 .' -\ .--= Y OH 3 4 a: X = H b X = SiMg c: X = SnR3 d: X = B(OR), Liebigs Ann. Chem. 1993. 711 -775

The conformations of a flexible n-heptane chain are considered. Six-membered ring annulation at the inner bonds leads to 1,3-disubstituted cyclohexanes, at the outer bonds to dicyclohexylmethane derivatives. Force-field calculations show that these molecules maintain the conformational mobility of the heptane chain to a large degree, yet some populate one particular conformer to more than 80% (up to 99% in the case of 17). The predominant conformers of the molecules described The shape of a molecular backbone, be it rigid or flexible, is responsible for the spatial disposition of any functional groups attached. When dealing with flexible molecular backbones, e.g. linear saturated hydrocarbon chains, the distance between the termini of the chain, or other attached functionalities, as well as their vectorial arrangement will be different for different conformers. The effective (time averaged) disposition of the chain ends, or of two attached functional groups, then depends on the conformer population, which is a function of the relative energy of the various conformers. A situation valuable for molecular design would be one in which a flexible chain has only one or two lowenergy conformations, which should be populated at least in excess of 80%. Such molecules will remain fully flexible, but nevertheless have in a weighted time average a .welldefined shape.Here we would like to discuss, to what extent the conformational equilibria of an n-heptane chain can be shifted by adding substituents in order to favor just one particular folding of the molecular backbone. The conformer populations of short-chain n-alkanes have been investigated by various computational techniques before: n-pentane[Z-61; nhexaner5]; ~-heptane [',~].The all-trans conformer, cf. 1, has been found to possess the lowest energy, followed by rotamers having an increasing number of gauche interactions. Rotamers having a g+g--sequence of dihedral angles, cf. 2, are among the highenergy conformations, which actually tend to relax the unfavorable syn-pentane interactions by opening of the dihedral angles[335971. But even these relaxed conformations remain of high energy, i.e. 1.4 to 3.0 kcal above the lowest energy all-trans conformation. Thus, molecules tend to avoid such g+g--conformations, which are destabilized by syn-pentane interactions [*]. This is the clue to favoring certain conformations of a molecular backbone: Substituents may be placed at the molecular backbone in such a way that they will cause unfavorable syn-pentane interactions between the substituents or with the backbone in all but one particular conformation. This will then by necessity be the only low-energy conformation populated.This principle is well-documented in many natural products, exhibiting flexible backbones, yet populating only few conformations of this backbone" Thus, by a judicious placement of substituents on a hydrocarbon chain, the number of available low energy conformations may be substantially Modelling of Various Conformations of a C,-ChainThe conformations of n...