Alkyne introduction

CHAPTER 6 Reactions of Alkynes Introduction to Multistep Synthesis... 

All possible stereoisomers of -4,5-dihydroxydec-2-enal 333 have been prepared from D- and L-arabinose, D-ribose and L-lyxose. Key reactions involve Cl Wittig homologation, reduction and C4,C5 oxidative cleavage. The eneyne 336, an intermediate for a synthesis of la,2S-dihydroxyvitamin D3, has been prepared from D-xylose via lactone 334 and 335. Lithioacetylene used for alkyne introduction and further elaboration provides 336 (Scheme 51). ... 

The intramolecular insertion of an internal alkyne into an aryl or alkenyl halide 727 generates aryl- or alkenylpalladium as an intermediate, which is trapped with an organozinc or organostannane to give 728. Overall cis addition to the alkyne takes place[595,596]. The reaction of the alkenylstannane 730 with the 2-bromomethylfuran 729 is used for the introduction of a prenyl group[597]. 

We conclude this introduction to hydrocarbons by describing the orbital hybridization model of bonding m ethylene and acetylene parents of the alkene and alkyne families respectively... 

Alkyne (Chapter 8 introduction) A hydrocarbon that contains a carbon-carbon triple bond, RC CR. 

The reaction of alkoxyarylcarbene complexes with alkynes mainly affords Dotz benzannulated [3C+2S+1C0] cycloadducts. However, uncommon reaction pathways of some alkoxyarylcarbene complexes in their reaction with alkynes leading to indene derivatives in a formal [3C+2S] cycloaddition process have been reported. For example, the reaction of methoxy(2,6-dimethylphenyl)chromium carbene complex with 1,2-diphenylacetylene at 100 °C gives rise to an unusual indene derivative where a sigmatropic 1,5-methyl shift is observed [60]. Moreover, a related (4-hydroxy-2,6-dimethylphenyl)carbene complex reacts in benzene at 100 °C with 3-hexyne to produce an indene derivative. However, the expected Dotz cycloadduct is obtained when the solvent is changed to acetonitrile [61] (Scheme 19). Also, Dotz et al. have shown that the introduction of an isocyanide ligand into the coordination sphere of the metal induces the preferential formation of indene derivatives [62]. 

Scheme 24 Introduction of the C - O and C - C linked alkyne handles at the C-3 position of the 2(lH)-pyrazinone scaffold...

Several ways to suppress the 2-oxonium-[3,3]-rearrangements might be envisioned. Apart from the introduction of a bulky substituent R at the aldehyde (Scheme 23) a similar steric repulsion between R and R might also be observed upon introduction of a bulky auxiliary at R. A proof-of-principle for this concept was observed upon by using of a trimethylsilyl group as substituent R in the alkyne moiety (Scheme 25, R = TMS). This improvement provided an efficient access to polysubstituted dihydropyrans via a silyl alkyne-Prins cyclization. Ab initio theoretical calculations support the proposed mechanism. Moreover, the use of enantiomerically enriched secondary homopropargylic alcohols yielded the corresponding oxa-cycles with similar enantiomeric purity [38]. 

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