Oxiranes alkynic

A diastereoselective tandem intramolecular hetero-5 + 2-cycloaddition/Claisen rearrangement of vinylic oxirane-alkyne substrates using a rhodium NHC complex to provide regiospecific and diastereoselective access to [3.1.0] bicyclic products has been reported (Scheme 153). ... 

The [5+2] cycloaddition has been included in several domino processes. For example, Feng and Zhang have recentiy developed a novel, regiospecific, and diastereoselective domino intramolecular hetero-[5+2] cycloaddition-Claisen rearrangement of vinylic oxirane-alkyne substrates 90, which employed the rhodium Af-heterocyclic carbene complex, RhCl(/-Pr)(cod), as catalyst [95]. The process provided the corresponding [3.1.0] bicyclic products 91 in moderate to high yields (47-92%), as shown in Scheme 20.39. The authors have demonstrated the complete chirality transfer by performing the reaction with an enantiomerically enriched... 

The addition of acetylides to oxiranes yields 3-alkyn-l-ols (F. Sondheimer, 1950 M.A. Adams, 1979 R.M. Carlson, 1974, 1975 K. Mori, 1976). The acetylene dianion and two a -synthons can also be used. 1,4-Diols with a carbon triple bond in between are formed from two carbonyl compounds (V. Jager, 1977, see p. 52). The triple bond can be either converted to a CIS- or frans-configurated double bond (M.A. Adams, 1979) or be hydrated to give a ketone (see pp. 52, 57, 131). 

The reaction of perfluoroalkyl iodides with alkenes affords the perfluoro-alkylated alkyl iodides 931. Q.a-Difluoro-functionalized phosphonates are prepared by the addition of the iododifluoromethylphosphonate (932) at room temperature[778], A one-electron transfer-initiated radical mechanism has been proposed for the addition reaction. Addition to alkynes affords 1-perfluoro-alkyl-2-iodoalkenes (933)[779-781]. The fluorine-containing oxirane 934 is obtained by the reaction of allyl aicohol[782]. Under a CO atmosphere, the carbocarbonylation of the alkenol 935 and the alkynol 937 takes place with perfluoroalkyl iodides to give the fluorine-containing lactones 936 and 938[783]. 

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-l, 2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5], The reaction can be explained by the transmetallation of the (7-allenylpailadium methoxide 4 with copper acetylides to form the allenyKalk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCb) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7]. 

Acidic cleavage of the oxirane ring in (210) results in formation of the alkynic aldehyde (211) (68TL4905). 

In an interesting catalysed conversion of trichloroethene by secondary amines into aminoacetamides, the initial steps are thought to involve the p-elimination of HC1 to produce dichloroethyne (Scheme 9.1), which reacts with the secondary amine under the wet conditions to produce the amide [35] the reaction does not work with N-alkylanilines. Such a mechanism is realistic, as it is well known [36] that trichloroethene is converted into the inflammable and explosive dichloroethyne by bases, and quaternary ammonium salts catalyse the formation of the alkyne when trichloroethene is reacted with oxiranes [37]. Chloroethynes have also been obtained by the catalysed reaction of terminal ethynes with carbon tetrachloride under basic conditions [38]. 

In recent years there has been a growing interest in the use of carbonyl ylides as 1,3-dipoles for total synthesis.127-130 Their dipolar cycloaddition to alkenic, alkynic and hetero multiple bonded dipolaro-philes has been well documented.6 Methods for the generation of carbonyl ylides include the thermal and photochemical opening of oxiranes,131 the thermal fragmentation of certain heterocyclic structures such as A3-l,3,4-oxadiazolines (141) or l,3-dioxolan-4-ones132-134 (142) and the reaction of carbenes or car-benoids with carbonyl derivatives.133-138 Formation of a carbonyl ylide by attack of a rhodium carbenoid... 

Transition-metal mediated carbene transfer from 205 to benzaldehyde generates carbonyl ylides 211 which are transformed into oxiranes 216 by 1,3-cyclization, into tetrahydrofurans 212, 213 or dihydrofurans 214 by [3 + 2] cycloaddition with electron-deficient alkenes or alkynes, and 1,3-dioxolanes 215 by [3 + 2] cycloaddition with excess carbonyl compound120 (equation 67). Related carbonyl ylide reactions have been performed with crotonaldehyde, acetone and cyclohexanone (equation 68). However, the ylide generated from cyclohexanone could not be trapped with dimethyl fumarate. Rather, the enol ether 217, probably formed by 1,4-proton shift in the ylide intermediate, was isolated in low yield120. In this respect, the carbene transfer reaction with 205 is not different from that with ethyl diazoacetate121, whereas a close analogy to diazomalonates is observed for the other carbonyl ylide reactions. 

---