Enynes skipped

The chiral 1,3-divinylallene (1,3,4,6-heptatetraene) (3) is obtained when the vinylace-tylene Grignard reagent 198 is first coupled to allyl bromide (199) and the resulting skipped enyne is subsequently isomerized under basic conditions (Scheme 5.29) [74]. 

Treatment of "skipped" enynes H CCH2CH=CHR with alkali amides presumably will lead to extensive isomerization into C>CCH=CHCH2R. With alkyllithium bases inverse addition has to be applied to avoid dimetallation. This isomerization reaction is likely to be even more serious in the case of the diynes HC=CCH2OCR, since the CH2-protons have kinetic acidities comparable with that of the ethynyl proton. 

Interaction between "skipped" enynes or diynes (RCH CHCT OCH or ROC--CH2C3CH) and alkali amides will lead to partial or complete isomerization (into RCH2CH=CHChCM or RCH=C=CHC=CM). Alkali amides in ammonia are unsuitable reagents if the compounds RSCT C IH and PhCH2C=CH are to be terminally mctallated (isomerization to RSCH=C=CH2 and PhCH OCHj). 

An iterative sequence of propargylation, enantioselective epoxidation of the resulting silylated skipped enynes, an endo selective hydroxyepoxide cyclisation and protodesilylation in which a SiMes group plays a pivotal role produces the tris-tetrahydropyran 9 in 18 steps <03OL2339>. The stereoselectivity of the endo selective oxacyclisations of 1,4-di- and... 

A large number of skipped systems, 1,4-enynes and 1,4-diynes, have been prepared by the reaction of alkynylmagnesium halides with allyl and propargyl halides, as illustrated in equations (139)-(144). Several of these products have served in the preparation of annulenes. These reactions are carried out in THF solution close to reflux temperature and in the presence of CuCl as catalyst. 

Lithium alkynylcuprates react with haloallenes to give similar skipped diacetylenes (see below). The related skipped enynes can be prepared by treatment of (pentadienyl)iron(tricarbonyl) halide complexes with dilithium trialkynylcuprates, the compounds being isolated as the iron(tricarbonyl)(diene) complexes (Scheme 4). Further examples of alkylation reactions of copper alkynides are illustrated in Scheme 5. Reaction between a lithium cyanoaikynecuprate and an iodoallene leads to a skipped diacetylene. This useful reaction has been used by Corey in his synthesis of hybridalactone (Scheme 6). °... 

Alkenes such as norbomene and styrene, and also 1,3-dienes add alkynylsilanes, the latter at the terminal double bond to afford branched skipped enynes. Cyanoalkynes split and add to alkynes and allenes to generate conjugated enynes. ... 

Z)-Alkenes. The method for generating (Z)-alkenes from alkynes can be extended to conjugated and methylene-skipped enynes. On quenching the titanacyclopropene intermediates with D O, dideuterio products are obtained. 

Addition of 9-BBN to alkynes is sufficiently slower than to similar alkenes to allow selective hydroboration-oxidation of skipped enynes (45) to produce 5,e-acetylenic alcohols (Scheme 22) without protection of the triple bond. ... 

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