Diynes stereochemistry

Rothwell and colleagues352 studied the titanium mediated [2 + 2 + 2] cycloaddition of alkenes with monoynes and diynes. Among the reactions studied, the reaction between styrene (29) and diyne 609 in the presence of titanium catalyst 610 proved cleanest (equation 175). The reaction yielded 614 via a [2 + 2 + 2] cycloaddition followed by a titanium mediated suprafacial [1,5] H-shift involving 611-613. The cis relationship between the trimethylsilyl group and the phenyl group indicated that the initially formed titananorbornene 611 had an endo stereochemistry. 

A stereochemical study of the synthesis of unsaturated 1,4-aminoalcohols via the reaction of unsaturated 1,4-alkoxyalcohols with chorosulfonyl isocyanate revealed a competition between an retentive mechanism and an SnI racemization mechanism, with the latter having a greater proportion with systems where the carbocation intermediate is more stable.254 An interrupted Nazarov reaction was observed, in which a nonconjugated alkene held near the dienone nucleus undergoes intramolecular trapping of the Nazarov cyclopentenyl cation intermediate.255 Cholesterol couples to 6-chloropurine under the conditions of the Mitsunobu reaction the stereochemistry and structural diversity of the products indicate that a homoallylic carbocation derived from cholesterol is the key intermediate.256 l-Siloxy-l,5-diynes undergo a Brpnsted acid-promoted 5-endo-dig cyclization with a ketenium ion and a vinyl cation proposed as intermediates.257... 

The pure E stereochemistry of the vinylsilane 21 clearly shows that the carbocylization proceeds in a totally stereoselective fashion to give, under these conditions (20 °C, 5 min) the configurationally stable (Z)-alkenylzinc bromide [21] 20 via a syn addition of the allenic zinc bromide to the acetylenic moiety. This carbocyclization represents a new desymmetrization of diynes such as 18 which serves to deliver functionalized exo-... 

Reaction between BTMSA and the diyne (31) in the presence of (r -C5H5)Co (CO)2 gives good yields of the polycyclic compound (32). The stereochemistry of the new ring junction is almost exclusively trans and this control of stereochemistry coupled with the simplicity and high yields (45—90%) of the reaction makes this a particularly attractive synthetic route. 

---