Pseudo-equatorial

Conversely, when A-alkyl tryptophan methyl esters were condensed with aldehydes, the trans diastereomers were observed as the major products." X-ray-crystal structures of 1,2,3-trisubstituted tetrahydro-P-carbolines revealed that the Cl substituent preferentially adopted a pseudo-axial position, forcing the C3 substituent into a pseudo-equatorial orientation to give the kinetically and thermodynamically preferred trans isomer." As the steric size of the Cl and N2 substituents increased, the selectivity for the trans isomer became greater. A-alkyl-L-tryptophan methyl ester 42 was condensed with various aliphatic aldehydes in the presence of trifluoroacetic acid to give predominantly the trans isomers. ... 

Conformational studies of c/5-6,11/)-H- and /rnns-6,ll/)-H-6-methyl-2,3,4,6,7,1 l/)-hexahydro-l//-pyrimido[6,l-n]isoquinolin-2-ones (136 and 137) by means of the MM2 method implemented in the HyperChem 4.5 suggested, that in the lowest-energy conformations heterocyclic moiety adopted trans-fu ed ring annotation in both cases, with a pseudo-equatorial and -axial methyl group, respectively (97LA1165). 

As a consequence of the pericyclic reaction path, the addition of a-stereogenic allylmctals to carbonyl compounds is accompanied by an effective 1,3-chirality transfer in the allylic moiety combined with 1,4-chira induction at the prostereogenic carbonyl group3032. The scheme also demonstrates the importance of the orientation of the substituent X in the six-membered transition state. By changing from a pseudo-axial to a pseudo-equatorial position, the cation-induced sy/i-attack addresses opposite faces of both prostereogenic moieties, leading to a Z-and an -isomer, these being enantiomeric in respect to the chiral moiety. 

Figure 7.23. (a) Pseudo-equatorial and pseudo-axial conformers of the dioxocarbenium ions, (b) ORTEP representations of the X-ray structures obtained for carbenium ion 49 and 50. (Taken from ref. 127.)... 

Phenylthio)nitroalkenes are also excellent intermediates for the synthesis of other heterocyclic ring systems. For example, tetrahydropyran carboxylic acid derivatives are formed by the intramolecular addition of oxygen nucleophile to l-(phenylthio)nitroalkene predominantly as the m-isomer (9.1 1) (see Eq. 4.40). The reaction may proceed via the chair-like transition state with two pseudo-equatorial substituents.50... 

We were mesmerized by the X-ray structure of hydroxy ketone 56 with its AB-ring junction being both. -hybridized, revealing a very different conformation from that of 48. The structure showed near-perfect chairs for the m-fused l-a /-decalinic. With this conformational preference, the p-Me group is pseudo-equatorial with the a-Me group now being pseudo-axial position. We believe this new a-Me orientation is responsible for the fact that the C5a-carbonyl is not reduced by NaBFLt it is sterically inaccessible, even by a nucleophile as small as a hydride. 

In addition to the [4+2] cycloaddition, intramolecular [2+2] photocycloaddition was also successfully used as a main procedure in the synthesis of (i)-ginkgolide B <00JA8453>. The studies on the model reactions and molecular mechanics calculation show that the stereochemistry of the substituents at C6 and C8 should influence severely the reaction diastereoselectivity. When syn-diastereomer 41 is subjected to irradiation the reaction gives a single diastereomer 42 in a quantitative yield since two substituents at C6 and C8 would be in pseudo-equatorial orientation in the chair-like transition state. 

These data demonstrate that the nitrogen atom in practically all cyclic nitroso acetals A or B, containing the isoxazolidine ring, deviates from the plane through the other four atoms (envelope). The nitrogen lone pair always pseudo-equatorial. 

The first total synthesis of (T)-arteannuin M was accessed using a tandem oxy-Cope/ene reaction.87 Divinylcyclohexanol 138 was heated in toluene and l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to give compound 140 as a single diastereomer in 55-60% yield (Scheme 28). The diastereoselectivity is controlled by the pseudo-equatorial position of the alkyl group in the transition state 139. 

A change in the allyl hapticity (q3 to p1 slippage) leading to the less substituted titanium-carbon o bond accounts for the observed y-regioselectivity. The anti diastereoselectivity stems from a pseudo-equatorial orientation of the aldehyde group. The diastereoselectivity of the reaction can be reversed through the use of a more coordinating cosolvent such as HMPA (Scheme 13.7) [14]. This reversal of anti to syn diastereoselectivity can be rationalized in terms of an open transition state. 

This protocol is also effective for the cyclization of an allenylaldehyde, the synthetic utility of which has been demonstrated in the synthesis of (+)-testudinariol A (Scheme 16.89) [97]. Cyclization of an allenylaldehyde provides a ris-cyclopentanol bearing a 2-propenyl group at the C2 position. The reaction mechanism may be accounted for by coordination of Ni(0) with both the aldehyde and the proximal alle-nyl double bond in an eclipsed fashion with a pseudo-equatorial orientation of the side chain, oxidative cyclization to a metallacycle, followed by Me2Zn transmetalla-tion and reductive elimination. 

Cyclic Esters of Phosphorous Acid.—A large number of 2-substituted-4-methyl-l,3,2-dioxaphospholans (88) have been prepared and their stereochemistry and conformations investigated by 1H and 31P n.m.r.69 Unlike the corresponding 1,3-dioxans, the tra/w-isomer (88a) is favoured in all cases, and each isomer is best described in terms of two rapidly equilibrating half-chair conformers with the 4-alkyl group pseudo-axial or pseudo-equatorial. 

The cyclobutanone 105, on thermolysis at 190°C, undergoes a retro-ene reaction yielding the crystalline 0,y-unsaturated ketone 115. This product in the solid regenerates 105 photochemically, in almost quantitative yield, whereas in solution the diketone 116 is the exclusive photoproduct. The ketone 115 is found in the solid to have the conformation 115a, a conformation that results, presumably, from the requirement for the bulky methyl at C-7 to adopt the pseudo-equatorial rather than the pseudoaxial position. This molecule in fact has a shape... 

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