Reagent substrate control

Stereoselective epoxidation can be realized through either substrate-controlled (e.g. 35 —> 36) or reagent-controlled approaches. A classic example is the epoxidation of 4-t-butylcyclohexanone. When sulfonium ylide 2 was utilized, the more reactive ylide irreversibly attacked the carbonyl from the axial direction to offer predominantly epoxide 37. When the less reactive sulfoxonium ylide 1 was used, the nucleophilic addition to the carbonyl was reversible, giving rise to the thermodynamically more stable, equatorially coupled betaine, which subsequently eliminated to deliver epoxide 38. Thus, stereoselective epoxidation was achieved from different mechanistic pathways taken by different sulfur ylides. In another case, reaction of aldehyde 38 with sulfonium ylide 2 only gave moderate stereoselectivity (41 40 = 1.5/1), whereas employment of sulfoxonium ylide 1 led to a ratio of 41 40 = 13/1. The best stereoselectivity was accomplished using aminosulfoxonium ylide 25, leading to a ratio of 41 40 = 30/1. For ketone 42, a complete reversal of stereochemistry was observed when it was treated with sulfoxonium ylide 1 and sulfonium ylide 2, respectively. ... 

Scheme 2. Substrate-controlled epoxidation of 12 and reagent-controlled epoxidation of 15.

Syntheses of nonracemic vinylaziridines by reagent- or substrate-controlled... 

Boland applied this methodology to Garner s aldehyde, and found the addition to be substrate-controlled rather than reagent-controlled (Scheme 9.13b) [68]. Viny-lepoxide 15 could thus also be obtained with high diastereoselectivity with achiral 9-MeO-9-BBN. 

In principle, asymmetric synthesis involves the formation of a new stereogenic unit in the substrate under the influence of a chiral group ultimately derived from a naturally occurring chiral compound. These methods can be divided into four major classes, depending on how this influence is exerted (1) substrate-controlled methods (2) auxiliary-controlled methods (3) reagent-controlled methods, and (4) catalyst-controlled methods. 

The substrate-controlled reaction is often called the first generation of asymmetric synthesis (Fig. 1-30, 1). It is based on intramolecular contact with a stereogenic unit that already exists in the chiral substrate. Formation of the new stereogenic unit most often occurs by reaction of the substrate with an achiral reagent at a diastereotopic site controlled by a nearby stereogenic unit. 

Double asymmetric synthesis was pioneered by Horeau et al.,87 and the subject was reviewed by Masamune et al.88 in 1985. The idea involves the asymmetric reaction of an enantiomerically pure substrate and an enantiomerically pure reagent. There are also reagent-controlled reactions and substrate-controlled reactions in this category. Double asymmetric reaction is of practical significance in the synthesis of acyclic compounds. 

Substrate control This refers to the addition of an achiral enolate (or allyl metal reagent) to a chiral aldehyde (generally bearing a chiral center at the a-position). In this case, diastereoselectivity is determined by transition state preference according to Cram-Felkin-Ahn considerations.2... 

At this juncture, it is useful to look at Table 7-1, in which the syntheses of erythronolide and the ansa chain are used as examples to show that reagent-controlled syntheses are clearly more advantageous than substrate-controlled reactions in terms of three criteria the overall yield, overall stereoselectivity, and number of steps involved in each of the syntheses. A careful examination of Table 7-1 clearly shows the advantages of this strategy. 

Addition of the indium reagent derived from the foregoing (P)-allenylstannane to /8-benzyloxy-a-methylpropanal as the aldehyde substrate at low temperature afforded a 70 30 mixture of anti,anti and anti,syn adducts (Eq. 9.141). The improved dia-stereoselectivity in this case can be attributed to substrate control, reflecting the chelating ability of an OBn versus an ODPS group. The lower temperature may also account for the improved diasteroselectivity. 

Intramolecular rhodium-catalyzed carbamate C-H insertion has broad utility for substrates fashioned from most 1° and 3° alcohols. As is typically observed, 3° and benzylic C-H bonds are favored over other C-H centers for amination of this type. Stereospecific oxidation of optically pure 3° units greatly facilitates the preparation of enantiomeric tetrasubstituted carbinolamines, and should find future applications in synthesis vide infra). Importantly, use of PhI(OAc)2 as a terminal oxidant for this process has enabled reactions with a class of starting materials (that is, 1° carbamates) for which iminoiodi-nane synthesis has not proven possible. Thus, by obviating the need for such reagents, substrate scope for this process and related aziridination reactions is significantly expanded vide infra). Looking forward, the versatility of this method for C-N bond formation will be advanced further with the advent of chiral catalysts for diastero- and enantio-controlled C-H insertion. In addition, new catalysts may increase the range of 2° alkanol-based carbamates that perform as viable substrates for this process. 

In recent decades, there have been extensive efforts made toward asymmetric PKRs. These efforts include the following categories (i) substrate-controlled asymmetric reactions and (ii) reagent-controlled asymmetric reaction. 

The aldol reaction between a chiral a-amino aldehyde 16 and an acetate derived enolate 17 creates a new stereogenic center and two possible diastereomers. Several different methods for the synthesis of statine derivatives following an aldol reaction have been reported most of them lead to a mixture of the (35,45)- and (3/ ,45)-diastereomers 18 (Scheme 3), which have to be separated by laborious chromatographic methods.[17 211 Two distinct approaches for stereochemical control have been used substrate control and reagent control. 

Hupe, E. Calaza, M. I. Knochel, P. Substrate-controlled highly diastereoselective synthesis of primary and secondary diorganozinc reagents by a hydroboration/B-Zn exchange sequence. Chem. Eur. J. 2003, 9, 2789-2796. 

The Paterson second-generation approach substantially reduced the total number of steps required to complete discodermolide. Notably, the use of chiral reagents and auxiliaries was completely eliminated, relying solely on substrate control to configure all the remaining stereocentres from the ubiquitous Roche ester (18), achieving a more cost-effective route. 

Research group Year Linear sequence Overall yield(%) Substrate- controlled stereocentres3 Reagent- controlled steieocentresb Chiral pool stereocentres ... 

Stereocentres configured by substrate-controlled reactions bStereocentres configured by reagent-auxiliary-controlled reactions Stereocentres accessed from chiral pool starting materials dData from Myles full disclosure in 2003 [52]... 

The racemic trialkylborane H should be produced in trace quantities only. Its formation is disfavored by both the reagent and the substrate control. 

As minor products we expect the racemic trialkylboranes F and/or G F is favored by reagent and disfavored by substrate control of stereoselectivity, whereas for G it is exactly the opposite. 

The condition for the occurrence of a mutual kinetic resolution is therefore that considerable substrate control of stereoselectivity and considerable reagent control of stereoselectivity occur simultaneously. 

For the discussion in Sections 3.4.4 and 3.4.5, we will assume ( ) that k6 > k7 that is, the reagent control of stereoselectivity is more effective than the substrate control of stereoselectivity. The justification for this assumption is simply that it makes additional thought experiments possible. These are useful for explaining interesting phenomena associated with stereoselective synthesis, which are known from other reactions. Because the thought experiments are much easier to understand than many of the actual experiments, their presentation is given preference for introducing concepts. 

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