Stereocontrolled epoxidation

The remaining chapters deal with a variety of catalysts for effecting oxidation reactions. Chapter 5 describes three simple protocols for the controlled oxidation of primary or secondary alcohols. The importance of stereocontrolled epoxidation and hydroxylation reactions is reflected by the fact that Chapter 6, directed at this field, is one of the most extensive sections of the book. An interesting example of an enantioselective Baeyer-Villiger reaction is featured in Chapter 7, together with an industrially important ketone to enone conversion. Oxidative carbon-carbon... 

Stereocontrolled epoxidation of acyclic systems. Stereoselectivity in the epoxi-dation of homoallylic alcohols is usually low. Nonetheless Kishi etal. have observed... 

In their synthesis of (+)-narciclasine 26, Elango and Yan performed a stereocontrolled epoxide formation via the bromohydrins 25. Following concomitant ring closure and N-piperonylation, treatment with catalytic SnCh affected intramolecular arene-epoxide coupling (Scheme 16) <2002JOC6954>. 

In general, the enantiomeric excess and the configuration of the optically active alcohols are strongly dependent on the structure of the starting carbonyl compound many examples of diastereo-selective reduction have also been reported. The reduction of an epoxy ketone is accompanied by a stereocontrolled epoxide hydrolytic opening to afford a racemic triol, diastereomerically pure (eq 4). ... 

The origin of dihydropyran 123 can be traced to the known dihydropyranone 124, derived from commercially available acetylacetaldehyde dimethylacetal 125. Although the oxygen at C4 of dihydropyran 123 is not present in the natural product, its role was to facilitate a stereocontrolled epoxidation reaction as well as the stereoselective formation of the Q-C-glycoside. Its presence also... 

The development of Mn-catalyzed epoxidation methods have provided useful methods for the stereocontrolled epoxidation of olefins. Evidence for a peroxy-Mn complex as a key intermediate in high valent manganese catalyzed epoxidations has been proposed <05JA17170>. 

It will be recognized by the reader that the aldol reaction has been one of the most intensely studied reactions (see Volume 2, Chapters 1.5-1.7) in contemporary organic chemistry, in terms of mechanism, stereochemical outcome and practical application, and therefore this work is relevant in the context of the Darzens condensation. Very recently, this technology has been applied to the Darzens condensation, making available a new asymmetric stereocontrolled epoxide synthesis, and this work is discussed in Section 1.13.7.2. 

Elango S, Yan TH. A short synthesis of (+)-narciclasine via a strategy derived from stereocontrolled epoxide formation and SnCU-catalyzed arene-epoxide coupling. J. Org. Chem. 2002 67 6954-6959. 

In a versatile stereocontrolled total synthesis of (+)-vinblastine, Fukuyama used a base-promoted macrocyclization of the N-nosylate and the terminal epoxide moiety present in 88 as one of the key steps, giving the 11-membered-ring product 89 in 82% yield (Scheme 8.23) [42],... 

Stereocontrolled nucleophilic opening at the C 1-position of epoxides to furnish substituted piperidine 182 was accomplished through the use of Lewis acids TMSOTf or Sc(OTf), <06CC2156>. 

The wide scope application of this transformation arises not only from the utility of epoxide compounds but also from the subsequent regiocontrolled and stereocontrolled nucleophilic substitution (ring-opening) reactions of the derived epoxy alcohol. These, through further functionalization, allow access to an impressive array of target molecules in enantiomerically pure form. 

R2Cu(CN)Li2 reaction with vie-epoxy mesylatesA higher-order cuprate reacts selectively with the epoxide group of the epoxy mesylate 1 to provide 2 with inversion at C3. Ring closure of 2 furnishes the epoxide 3, which reacts with a second equivalent of the higher-order cuprate to furnish meso-4, with inversion at both C, and C3. This two-step reaction provides a route to acyclic alcohols with useful stereocontrol at both adjacent centers. 

Aryl and alkenyl groups undergo this anft -migration more easily than an alkyl group. This rearrangement in combination with Sharpless asymmetric epoxidation affords a stereocontrolled route to aldols and 1,3-diols (second example). 

As an example of the usefulness of the Sharpless asymmetric epoxidation the enantioselective synthesis of (-)-swainsonine and an early note by Nicolaou on the stereocontrolled synthesis of 1, 3, 5...(2n + 1) polyols, undertaken in connection with a programme directed towards the total synthesis of polyene macrolide antibiotics, such as amphotericin B and nystatin Aj, will be discussed. 

A fully stereocontrolled preparation of 499 has recently been completed by Paquette and coworkers When triene 508 was treated with /7-toluenesulfonyl-acetylene, highly stereoselective addition from the endo surface occurred to deliver an adduct which was directly epoxidized (Scheme XLI). The proximity of the two jc bonds in 509 allows for ready photocyclization. Oxidative cleavage of 510 afforded... 

It is noteworthy that the chlorohydrin 27 and acetoxy alcohol 28 can be synthesized in a highly stereocontrolled manner. Zr-catalyzed reaction proceeds, contrasting with the Sn—catalyzed reaction, via an epoxide intermediate to give the acetoxy alcohol. 

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