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37, ,
such reactions include transition metal catalyzed ene-type cycloisomerizations, 38 39 [4+2] and
[5+2] cycloadditions 40 and [2+2+1] cyclocarbonylation 41 reactions of enynes (Scheme 1.7).
Aside from increasing molecular complexity, an important aspect of these reactions is that metal
catalysis often allows for bond formation that would be difficult or impossible under
conventional methods, to readily occur under mild conditions. A typical example is the
intramolecular [4+2] cycloaddition of electronically unactivated dieneynes proceeding under
Rh(I) catalysis (27 to 30, Scheme 1.7). 42
Scheme 1.7 Some carbocyclization reactions of 1,6-enynes.
33
32
R 1
R 1
O
enyne
metathesis
CO
[2+2+1]
R1 28
R 1 = H or alkyl
27
31
R 1
R 1 =
[5+2]
R 1 = alkyl
The vast potential for increasing molecular complexity and achieving skeletal diversity
via metal catalyzed carbocyclization reactions (e.g., cycloadditon, cycloisomerization,
cyclocarbonylation, etc.) remains an untapped resource for DOS. One of the few examples of
utilizing a Pauson-Khand cyclocarbonylation reaction in the synthesis of a library of amino-acid
derived bicyclic skeletons 35 was reported by Bolton in 1997 (Scheme 1.8). 43 More recently, a
cyclocarbonylation reaction was also employed to prepare a library of fused tricyclic systems 37
shown in Scheme 1.8. 44
11
R 1 =
[4+2]
29
30
R 2