BALDWIN'S RULES

'Baldwin's Rules' in organic chemistry are a series of guidelines outlining the relative favourabilities of ring closure reactions in alicyclic compounds. They were first proposed by Jack Baldwin in 1976 ^{[1] [2]}.
The rules classify ring closures in three ways:

★ the number of atoms in new ring formed

★ into 'exo' and 'endo' ring closures, depending whether the bond broken during the ring closure is inside (endo) or outside (exo) the ring which is formed

★ into 'tet', 'trig' and 'dig', depending whether the electrophilic carbon is tetrahedral/sp3 (tet), trigonal/sp2, (trig) or digonal/sp, (dig)
Thus, a ring closure reaction could be classified as, eg. 6-exo-trig
:
Baldwin discovered that orbital overlap requirements for the formation of bonds favour only certain combinations of ring size and the exo/endo/dig/trig/tet parameters.
It should be noted that Baldwin's rules are general guidelines, and that exceptions do occur. The 'Baldwin disfavoured ring closures' and 'Baldwin favoured ring closures:' are listed below.

		''tet''	''trig''	''dig''
3	''exo''	√	√	X
	''endo''		X	√
4	''exo''	√	√	X
	''endo''		X	√
5	''exo''	√	√	√
	''endo''	X	X	√
6	''exo''	√	√	√
	''endo''	X	√	√
7	''exo''	√	√	√
	''endo''		√	√

Contents

Examples

References

Examples

In one study seven-membered rings are constructed in a tandem 5-exo-dig addition reaction / Claisen rearrangement ^[3]:
:
A 6-endo-dig pattern is observed in a allene - alkyne 1,2-addition / Nazarov cyclization tandem catalysed by a gold compound ^[4]:
:

References

1. Baldwin, J. E., Rules for Ring Closure, ''J. Chem. Soc., Chem. Commun.'' '1976', 734.
2. Baldwin, J. E., et al, Rules for Ring Closure: Ring Formation by Conjugate Addition of Oxygen Nucleophiles, J. Org. Chem., '42'(24), 3846 (1977).
3. ''Synthesis of Seven-Membered Carbocyclic Rings via a Microwave-Assisted Tandem Oxyanionic 5-exo dig Cyclization-Claisen Rearrangement Process'' Xin Li, Robert E. Kyne, and Timo V. Ovaska J. Org. Chem., 72 (17), 6624 -6627, '2007'.
4. ''Gold-Catalyzed Synthesis of Bicyclo[4.3.0]nonadiene Derivatives via Tandem 6-endo-dig/Nazarov Cyclization of 1,6-Allenynes'' Guan-You Lin, Chun-Yao Yang, and Rai-Shung Liu J. Org. Chem. '2007', 72, 6753-6757