Sequential treatment of (Z)-3-methyl-2-penten-4-ynal
(10) with the allenylborane 2 and
2-aminoethanol furnished 11. The use of 11 for
cross-coupling with aryl iodides followed by the
KH-induced
syn elimination of the coupled adducts 13 provided easy
access to a variety of dienediynes 14 for
subsequent conversions to
dibenzo[a,e]cyclooctenes
(sym-dibenzocyclooctatetraenes) 17. By
cross-coupling with 1,2- and 1,4-diiodobenzene, it was possible to obtain
19 and 25 having two dienediynyl
moieties. It was anticipated that the oligomers 21 and
26 containing multiple
dibenzo[a,e]cyclooctenyl units could thus be synthesized. The 1H NMR
spectrum of 32 having three
dibenzo[a,e]cyclooctenyl units indicates the presence of three diastereomers
32a−c in a random statistical
distribution of 2:1:1. Similarly, the 1H NMR spectrum
of 33 having two
dibenzo[a,e]cyclooctenyl
units exhibits signals that support the presence of two diastereomers
33a and 33b in a 1:1 ratio.