Regioselectivity from elimination reactions

The elimination is promoted by oxidation of the addition product to the selenoxide by f-butyl hydroperoxide. The regioselectivity in this reaction is such that the hydroxy group becomes bound at the more-substituted end of the carbon-carbon double bond. The regioselectivity of the addition step follows Markovnikov s rule with PhSe+ acting as the electrophile. The elimination step specifically proceeds away from the oxygen functionality. 

In contrast, the regioselectivity of the reaction with acetoacetaldehyde dimethylacetal under the same conditions is poor, resulting in 67% 5-methyl-TP (89JHC1393). In the presence of sodium ethoxide, the reaction leads to 5-methyl-TP exclusively. Here, in the present authors opinion, the regioselection may be controlled by the reaction of the AT anion with the product of base-induced methanol elimination from the keto acetal. 

Reactions of 99 with 1-substituted dienes gave a mixture of two different cyclohexadienes 103 and 104, resulting from the non-regioselective sulfinyl elimination from the unstable adducts 102. The optical purities of the conjugated cyclohexadienes 103 (ee >96%) are very high regardless of the catalyst used,... 

An N-alkyl protecting group is crucial to the regioselectivity of this reaction with N-aryl groups products 333 are obtained, arising from 6-endo cyclisation followed by elimination. 

The selenosulfonates (26) comprise another class of selenenyl pseudohalides. They are stable, crystalline compounds available from the reaction of selenenyl halides with sulftnate salts (Scheme 10) or more conveniently from the oxidation of either sulfonohydrazides (ArS02NHNH2) or sulftnic acids (ArS02H) with benzeneseleninic acid (27) (equations 21 and 22). Selenosulfonates add to alkenes via an electrophilic mechanism catalyzed by boron trifluoride etherate, or via a radical mechanism initiated thermally or photolytically. The two reaction modes produce complementary regioselectivity, but only the electrophilic processes are stereospecific (anti). Similar radical additions to acetylenes and allenes have been reported, with the regio- and stereochemistry as shown in Scheme 11. When these selenosulfonation reactions are used in conjunction with subsequent selenoxide eliminations or [2,3] sigmatropic rearrangements, they provide access to a variety of unsaturated sulfone products. 

Structure 4 is an intermediate for manufaeturing vitamin A (Scheme 2). The annual demand for vitamin A is about 3000 tons. Major producers are BASF, Hoffmann-La Roche and Rhone-Poulenc Animal Nutrition [55]. At an early stage in the synthesis BASF and Hoffmann-La Roche are using a hydroformylation step to synthesize 4 starting from l,2-diacetoxy-3-butene (5) and 1,4-di-aeetoxy-2-butene (6), respectively [56, 57]. The selectivity toward the branched product in the BASF process is achieved by using an unmodified rhodium carbonyl catalyst at a high reaction temperature. The symmetry of 6 in La Roche s process does not lead to regioselectivity problems. Elimination of acetic acid and isomerization of the exo double bond (La Roche) yields the final product 4 in both processes. 

In the saussurea lactone synthesis by Grieco (Scheme 31) [18], cleavage of the C(2)-C(3) link was carried out by ozonolysis of the C(2)-C(3) double bond in compound 247, obtained regioselectively from tetrahydrosantonin 3b by a Shapiro reaction, to give diol 51. Subsequent elimination of both hycftoxyl groups in several steps yielded saussurea lactone (56). An improvement of tWs procedure, described by our group [17], was based on the transformation of diol 51 into diselenide 52, which could then be transformed directly into saussurea lactone (56) (see Scheme 8). 

SgZ fashion (labeled in structure 22) leads to the chelated rr-allylpalladium intermediate 22, which can potentially lose either of two protons (labeled H], and in structure 22) to afford either 18 or 19, respectively. From recent work in the literature, notably that of Krafift and co-workers,it is recognized that chelated rr-allylpalladium intermediates can show unusual regioselectivity in addition reactions. Perhaps related factors govern the regioselective loss of to afford intermediate 23 and, ultimately, the enediene product 19 there is evidence suggesting that loss of proton H, occurs via an E2 rather than /S-hydride elimination pathway. 

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