Epoxidation propargyl epoxide

The preparation of other 6-aIkylated steroids has also been accomplished with 5,6-epoxides. Thus treatment of 3,3-ethylenedioxy-5a,6a-epoxy steroids (6) with acetylenedimagnesium bromide,propargylmagnesium bromide " or phenylmagnesium bromide, gives the corresponding 6 -ethynyl-, 6j5-propargyl- and 6/ -phenyl-3,3-ethylenedioxy-5a-ols. 

In mosl allylation reactions, only a catalytic amount of CuCN-2LiCl is required [41]. Use of die chiral ferrocenylamine 104 as a catalyst makes enables asymmetric allylation of diorganozinc reagents to be effected witli allylic chlorides iScbeme 2.3G) [78]. Related electropb des such as propargylic bromides [79] and unsaturated epoxides [80] also undergo Su2 -substitution reactions iScbeme 2.37). 

The reaction of propargylic epoxide 4 with excess amounts of dialkylcuprates gives a significant amount of reduction byproduct 6, but the use of RCu(CH2SOCHa)Li affords a-hy-droxyallene 5 in high yield94. 

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

The method is quite useful for particularly active alkyl halides such as allylic, benzylic, and propargylic halides, and for a-halo ethers and esters, but is not very serviceable for ordinary primary and secondary halides. Tertiary halides do not give the reaction at all since, with respect to the halide, this is nucleophilic substitution and elimination predominates. The reaction can also be applied to activated aryl halides (such as 2,4-dinitrochlorobenzene see Chapter 13), to epoxides, " and to activated alkenes such as acrylonitrile. The latter is a Michael type reaction (p. 976) with respect to the alkene. 

The stereochemistry of the first step was ascertained by an X-ray analysis [8] of an isolated oxazaphospholidine 3 (R = Ph). The overall sequence from oxi-rane to aziridine takes place with an excellent retention of chiral integrity. As the stereochemistry of the oxirane esters is determined by the chiral inductor during the Sharpless epoxidation, both enantiomers of aziridine esters can be readily obtained by choosing the desired antipodal tartrate inductor during the epoxidation reaction. It is relevant to note that the required starting allylic alcohols are conveniently prepared by chain elongation of propargyl alcohol as a C3 synthon followed by an appropriate reduction of the triple bond, e. g., with lithium aluminum hydride [6b]. 

Ni(cyclam)]2+ was shown to be an efficient electrocatalyst for the intramolecular cyclization-carboxylation of allyl or propargyl-2-haloaryl ethers,200 and for the synthesis of cyclic carbonates from epoxides and carbon dioxide.201... 

The effectiveness of dimethyl sulfide as an additive for the selective formation of anti-product 22 from propargyl epoxide 20 may be due to the formation of stabilized copper species, which are less prone to undergo electron transfer processes. In this respect, other soft ligands which bind strongly to copper, in particular phosphines and phosphites [8h-j, 25, 28], have been used even more frequently. These additives also serve to suppress the formation of a common side product, i.e. an allene containing a hydrogen atom instead of the carbon substituent which should... 

With the proper choice of reaction conditions, diastereoselective synthesis of a-allenic alcohols 69 and 70 from propargylic epoxide 68 was achieved [80, 81], With RMgBr and 5 mol% of CuBr/2PnBu3, anti allenic alcohols 69 are obtained with up to 100% diastereoselectivity. On the other hand, syn allenic alcohols 70 can be prepared with 88-96% diastereoselectivity with RMgCl, Me3SiCl and 5mol% CuBr (Scheme 3.36). 

Ring-Opening Reactions of Propargyl Epoxides and Related Compounds... 

Not least for the syntheses of natural products, alkoxycarbonylations with formation of allenic esters, often starting from mesylates or carbonates of type 89, are of great importance [35, 137]. In the case of carbonates, the formation of the products 96 occurs by decarboxylation of 94 to give the intermediates 95 (Scheme 7.14). The mesylates 97 are preferred to the analogous carbonates for the alkoxycarbonylation of optically active propargylic compounds in order to decrease the loss of optical purity in the products 98 [15]. In addition to the simple propargylic compounds of type 89, cyclic carbonates or epoxides such as 99 can also be used [138]. The obtained products 100 contain an additional hydroxy function. 

Five- and six-membered cyclic allenylidenesiloxanes have been prepared by internal displacements of siloxane epoxides by lithiated propargylic siloxanes (Eqs. 9.49-9.51) [58], The mode of attack is related to the stereochemistry of the epoxide. A trans epoxide (Eq. 9.49) give rise to a six-membered siloxane by a 6-endo pathway, whereas cis epoxides (Eqs. 9.50 and 9.51) undergo 5-exo cleavage. 

Scheme 2.37. Substitution reactions of propargylic bromides and unsaturated epoxides with organozinc reagents.

Scheme 3.43. Reactions of a-aminoalkylcuprates with allylic epoxides [171] and propargylic substrates (Boc = t-butoxy-carbonyl) [172].

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