Sharpless reaction modification

The reaction modification featuring titanium (III) as the starting point was originated somewhat later by Barry Sharpless. The mechanism is shown under (c) and is more speculative than (a) and (b). We hoped that these titanium (II) species could be made, and we are now certain that they are real and available entities. For example, titanium di-benzoxide can be isolated and shown to yield bibenzyl on thermolysis. 

The preparation described here is a slight modification of a route published by King and Sharpless via the osmium-catalyzed asymmetric dihydroxylation (AD) reaction of 1 -phenyl-1-cyclohexene. The major strengths of this process are that either enantiomer can be prepared in high optical purity (> 99.5% ee) v/ithout the need for chromatography. 

The absolute configurations of the new stereogenic centers in 15, introduced through the Sharpless epoxidation and crotylation, can be predicted with the aid of the rules cited above. It is worth notice, however, that crotylation in the present case was carried out at 10 kbar. This is a frequently used modification of standard reaction conditions (with other synthetic methods as well) designed to improve selectivity and/or yield. 

Complexes based on titanium excess tartrate combination (the Padova system). In 1984, the same year the Orsay group developed their system, a group in Padova, Italy, headed by Modena,47 developed a different system, able to oxidize sulfides to sulfoxides with high selectivity, also based on a modification of the Sharpless catalyst. The Padova group used TBHP in the presence of 1 mol equiv of Ti(0-/-Pr)4/(/ ,/ )-DET, 1/4 combination. The reactions were performed at -20... 

A useful modification of selenium dioxide allylic oxidation was introduced by Sharpless in 1977. An inevitable complication of the normal reaction is the production of odoriferous low-valent selenium species which may be difficult to remove from the product mixtures and which can give rise to organo-selenium by-products. Sharpless provided a solution to this problem by the introduction into tlw re-... 

An original modification of the Sharpless AA reaction using excess of N-bromoacetamide as nitrogen/bromine source appeared particularly useful method for the transformation of 2-aryl-vinylphosphonates (338) into syn-2-aryl-2-amino-l-bromoethyl phosphonates (339) (Scheme 91). ... 

In 1996, Sharpless reported that modification of the AD reaction, by the inclusion of a nitrogen source, which also functions as an oxidant, gives an asymmetric aminohydroxylation (AA). > ... 

The advent of azide/alkyne click chemistry, reintroduced by Sharpless and Meldal earlier this century, has prompted an avalanche of publications in the fields of biochemistry, material science and biopolymers. As of 2008, more than 1000 publications on this subject are listed on the Sharpless website. I have noticed that gradually other [3+2] and [4+2] cycloaddition reactions are included, indicating that cycloaddition chemistry is useful for construction and modification of biopolymers. Especially, the [3+2] Huisgen chemistry is useful because in addition to azides many other 1,3-dipolar species react with dipolarophiles at room temperature and the yields often approach quantitative. There is also renewed interest in [4+2] Diels-Alder chemistry. 

The introduction of the concept of Click chemistry, as a family of organic reactions that fulfil certain criteria drawn by Sharpless and coworkers in 2001 [194], has indeed captured the attention of synthetic chemists in the field of postpolymerization modification towards glycopolymer synthesis [32]. The most widely employed Click reaction is the CuAAC reaction. ATRP has been used extensively in conjunction with CuAAC Click chemistry. This is probably because both techniques are mediated by Cu(I). Moreover, the halogen chain ends of polymers prepared using ATRP can easily be transformed into azides to form what is known as azido-telechelic polymers. Many examples of glycopolymers prepared by the combination of Click chemistry and ATRP have been reported [32, 99]. 

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