Chelation-controlled process

Another example of this process is delineated in equation (19). In this case, chiral imine (53) was found to react under Lewis acid catalysis with diene (54) to provide adduct (55) stereoselectively via a Cram chelation controlled process. This cycloadduct was transformed in a few steps to amino sugar (56). 

An example of this type of effort is as follows. Complexation of the carbonyl oxygen of N/(-dibenzyl-a-amino aldehydes wiA Lewis acids such as BF3, ZnBr2 or SnCU, followed by ad tion of trimethylsilyl cyanide leads to adducts formed by a nonchelation-controlled process. Complexation with TiCU or MgBr2 affords the opposite stereochemistry preferentially, tl ugh a chelation-controlled process (Scheme 1). ... 

Matched and mismatched characteristics have been observed in reactions with non-racemic a-benzyloxypropionaldehyde. The matched asymmetric allylation of (E)-stannane 278 with (S)-aldehyde, initiated by complexation with BFs OEta, exclusively provides the E-4.5-syn-5,6-anti compound 279 as the expected Felkin-Anh adduct (Scheme 5.2.61, top). On the other hand, the Q -chelation-controlled process can also be achieved via a matched case of double diastereoselection using the (S)-stannane 280 and pre-complexation with MgBr2 OEt2. The syn product 281 is rationalized via the antiperiplanar transition state 282 (Scheme 5.2.61, bottom). 

Felkin model for asymmetric induction if one uses the order of ligand preferences for the anti position MeO > Bu > Ph > Pr > Et > Me > H. Note that the major isomers produced in reactions of the a-meth-oxy aldehydes (Table 17, entries 5-9) are not those expected from a chelation-controlled process. 

Auxiliaries have been designed to exploit the high selectivities of chelation-controlled processes in asymmetric synthesis. Among these are the oxathiane [35,47-50]... 

A high degree of stereoselectivity can be achieved in chelation-controlled reactions, utilizing hydroxy groups as stereodirectors (Eq. 59) [69], Studies have revealed that non-chelation-controlled processes may also proceed with enhanced selectivities, and this led to the synthesis of the cA-decalin skeleton of vinigrol (Eq. 60) [70]. 

The four isomeric 6,7-unsaturated octuronate derivatives 34 (see Vol. 25, Chapter 2, ref. 25 fcM their preparation) have been transformed to the octose noonoacetonides 35 and octitol octaacetates 36, some of which are new compounds. Vanadium (II)-catalysed pinacol cross condensation of the D-xylo-md D-ga/acro-dialdehyde derivatives 37 and 39 with paiafonnaldehyde furnished the d-configurated products 38 and 40, respectively, in a non-chelation controlled process, together with very minor proportions of the corresponding L-sugars."... 

There has been great interest in the synthesis of taxanes, as a consequence of the important anticancer properties of taxol, the best known member of this class of compounds. In Holton s landmark synthesis of (-)-taxusin (56), a number of interesting observations characteristic of the general aspects of chelation-controlled processes can be made (Scheme 2.6) [65]. The addition of 1-lithio-l-methoxyethene to 54 proceeds optimally in hexane to furnish adduct 55 with >30 1 diastereoselectivity. 

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