Page 347 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Scheme 4.4. (Continued) 319

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Entry 2 is a 5-exo bromocyclization. The reaction in Entry 3 involves formation of a
-lactone in an acyclic system. This reaction was carried out under conditions that
lead to the thermodynamically favored trans isomer. Entry 4 shows typical iodolac-
tonization conditions and illustrates both the anti stereoselectivity and preference for
formation of five-membered rings. In Entry 5, a six-membered lactone is formed,
again with anti stereospecificity. Entry 6 is a cyclization of a t-butyl carbonate ester.
The selectivity between the two double bonds is the result of the relative proximity
of the nucleophilic group. Entry 7 is a closely related reaction, but carried out at a
much lower temperature by the use of IBr. The cis:trans ratio was improved to nearly
26:1. The ratio was also solvent dependent, with toluene being the best solvent. Entry
8 is a variation using a lithium carbonate as the nucleophile. Entries 9 and 10 involve
hydroxy groups as nucleophiles. Entry 9 is a 6-endo iodocyclization. In Entry 10,
a primary hydroxy group serves as the nucleophile. Entry 11 is another cyclization
involving a hydroxy group, in this case forming a 7-oxabicyclo[2.2.1]heptane structure.
Entry 12 is a rather unusual 5-endo cyclization.
Entry 13 shows cyclization with concomitant loss of the benzyloxycarbonyl group.
The TS for this reaction is 5-exo with conformation determined by the pseudoequatorial
position of the methyl group.

CH 3
2
PhCH 2 O O CO CH 3
I +
N
H
CH 3
Entry 14 involves formation of a lactam by cyclization of a bis-trimethylsilylimidate.
The stereoselectivity parallels that of iodolactonization.
Entries 15 to 18 are examples of use of iodocyclization in multistep syntheses.
In Entry 15, iodolactonization was followed by elimination of HI from the bicyclic
lactone. In Entry 16, a cyclic peroxide group remained unaffected by the standard
iodolactonization and subsequent Bu SnH reductive deiodination. (See Section 5.5 for
3