Selected rearrangements of alkynes to allenes

The process is illustrated by the conversion of 3-chloro-3-methylpent-l-yne into 3-methylpenta-1,2-diene (Expt 5.21). Isolation of the allene is facilitated by 

An example of this reaction is provided by the conversion of 2-methylbut-3-yn-2-ol to l-bromo-3-methylbuta-l,2-diene (Expt 5.22). 

Secondary propargyl alcohols (17 R2 = H) require rather stronger acid and a longer reaction time, and the resulting bromoallenes may contain up to 5 per cent of the 3-bromoacetylenes, which may, however, be removed by fractional distillation. 

3-Chloro-3-methylpent-l-yne. Place 73.6g (0.75 mol) of 3-methylpent-1 -yn-3-ol and 750ml of concentrated hydrochloric acid (d 1.18) in a 2-litre separating 

0— H stretching vibration of the alcohol, has completely disappeared (about 15 minutes). Discard the dark lower aqueous acid layer and dry the pale brown organic layer over anhydrous potassium carbonate for about 2 hours a fresh portion of anhydrous potassium carbonate should be added after 30 minutes. Remove the drying agent by filtration. The yield of deep straw-coloured liquid, which consists of 3-chloro-3-methylpent-l-yne (c. 73%) and 

---

Cationic gold(I) complexes favor the formation of six- and seven-membered rings by 6-endo-dig, 6-exo-dig, and 1-exo-dig cyclization. However, indoloazo-cines V are selectively obtained with AuCls via %-endo-dig cyclization. Internal alkynes are also active in the intramolecular process leading to allenes VI and tetracyclic compounds VII (Scheme 1.10). In Scheme 1.10, the proposed mechanism for the formation of the different products is shown. Nucleophilic attack of the indole on the activated alkyne affords intermediate VIII, which arises from a 1,2-shift of the initially formed seven-membered ring iminium cation. Proton loss from VIII forms azocine V, while protonation of intermediate VIII leads to an open intermediate IX, which rearranges to the final allene VI or the tetracyclic compound VII via Michael-type addition of the XH group in intermediate X. 

---