A General Strategy for the Stereocontrolled Preparation of Diverse 8- and 9-Membered Laurencia-Type Bromoethers
Scott A. Snyder, Daniel S. Treitler, Alexandria P. Brucks, and Wesley Sattler
This time some cool methodology from the Snyder group involving the use of a recently reported new reagent: BDSB. It is formed by the reaction of diethylsulfide, SbCl5 and bromine:
With this reagent a lot of bromonium ion induced cyclization reactions are possible which do not work well with the common reagents e.g. NBS or TBCO. In a communication from 2009 the group used this reagent quite efficiently to produce fused cyclohexane systems.
All these reactions were conducted with BDSB in nitromethane. No or very low yields of the products were obtained using common reagents. Encouraged by these results the group conducted some experiments to form larger ring systems in a biomimetic manner:
As can be seen from scheme 3 some quite interesting motifs can be produced in a highly selective and efficient way. Recently the group reported an extension of this methodology which prompted me to write this little review.
They used BDSB to convert tetrahydropyrans into oxocane ring systems through an interesting biomimetic rearrangement reaction.
By exposing the substituted THP-rings to BDSB a bromonium ion induced cyclization occurred which opens the five membered ring to an eight membered one. And all this in a stereoselective manner with high ee’s. Following this approach some members of the lauroxocane group of natural products were produced.
Depending on the tetrahydropyran used a lot of diastereomers can easily be synthesized. In a representative example the group started from pentenol and methoxypropene to produce via a Claisen rearrangement 5-octenone. The second fragment derived from hexanal which was stereoselectively chlorinated using NCS and L-proline. An aldol reaction combined both halves and the resulting aldol product was exposed to anti selective reduction conditions. Cyclization to the tetrahydropyran was accomplished under high pressure in methanol.
I think this is a very useful methodology to form medium sized rings otherwise not so easy to access. Because of the ease of preparing BDSB it will hopefully find more applications in literature and total synthesis.