Baldwin's rules

, 42(24), 3846 (1977). doi|10.1021/jo00444a011] .

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. 5-exo-trig. It should be noted that 5-endo-trig is disfavoured, but look "nice" on the paper:

:

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. Eg. cations often disobey Baldwin's rules, and reactions where a second-row atom are included in the ring.

The Baldwin disfavoured ring closures and Baldwin favoured ring closures: are listed below.

Examples

In one study seven-membered rings are constructed in a tandem 5-exo-dig addition reaction / Claisen rearrangement ["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. DOI|10.1021/jo0710432] :

:

A 6-endo-dig pattern is observed in a allene - alkyne 1,2-addition / Nazarov cyclization tandem catalysed by a gold compound ["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 DOI|10.1021/jo0707939] :

:

References