|Scheme 1. Taken from Palomo's publication|
In Phillips and Pfeiffer's paper, the endo-acrylisoborneol is then reacted with alkene 6 via a metathesis using Grubb's catalyst (7). The product is a substrate (8) which is next used as a Diels-Alder precursor.
|Conditions: (1) 5 mol % Grubbs catalyst (7), 93%|
The precursor 8 was then reacted with 1,4-dimethylcyclohexadiene in acidic media (2 equiv TfOH) at -78 °C provided the Diels-Alder product (9, a bicyclo[2.2.2]octene).
|Conditions: (2) 1,4-dimethylcyclohexadiene, TfOH (2 equiv), -78 °C, 70%|
|The cyclization between 8 and 1,4-dimethylcyclohexadiene occurs through a Diels-Alder cyclization.|
Reacting product 9 with CAN (Cerium Ammonium Nitrate), removes the borneol group through a radical reaction. This procedure is also described by Palomo et al. in an earlier publication.
|Conditions: (3) CAN, aqueous MeCN, 82%.|
The carboxylic acid and pivalic ester are thereafter reduced to the alcohols using LiAlH4. The product is then subjected to a Swern oxidation to yield the corresponding aldehydes.
|Conditions: (4) a) LiAlH4, THF, reflux, 16h, 99%. b) (COCl)2, DMSO, Et3N, 84%|
using Dess-Martin periodinane, providing the bis-enone 12. The bis-enone 12 was then exposed to Grubb's catalyst 7, under an atmosphere of ethylene, providing 13.
|Conditions: (5) a) Vinylmagnesium bromide, CeCl3. b) Dess-Martin Periodinane. (6) 20 mol% 7, ethylene, Toluene|
So, I did not quite get it at the beginning what was going on between 12 and 13, but I tried to make a drawing to understand it better. Here it is:
And it is very late right now so I will pause here and continue tomorrow or when I have some more time to write.
Peace & Luv