Diastereoselective reductive aldol reactions of Boc-protected electron deficient pyrroles

The partial reduction of electron-deficient pyrroles: procedures describing both Birch (Li/NH3) and ammonia-free (Li/DBB) conditions

The partial reduction of electron-deficient pyrroles using either Birch (Li/NH(3)) or ammonia-free (Li/di-tert-butyl biphenyl) conditions allows formation of pyrroline compounds in good yield and, when combined with a reductive alkylation or similar approach, leads to highly functionalized, synthetically useful compounds. This methodology has been proven in the syntheses of several complex natural products, all of which show interesting biological activity. This protocol describes in detail the following stages of the partial reduction procedure: formation of the reducing solution, partial reduction of the pyrrole compound and finally quench of the resulting anion/dianion using either protonating agents or an aldehyde. The ammonia-free conditions described herein are particularly useful for reactions requiring the use of reactive electrophiles, such as acid chlorides or enolizable aldehydes, which are incompatible with the standard Birch reduction conditions. The reaction procedure for the ammonia Birch reduction (procedure A) takes about 9.5 h to complete. Those described for the ammonia-free reductions, procedure B and procedure C, can be expected to take approximately 33 and 8 h, respectively.

Synthesis of the Pyrrolidinone Core of KSM-2690 B

[reaction: see text] The first synthesis of the pyrrolidinone core of the polyene beta-lactone antibiotic KSM-2690 B is described. An ammonia-free Birch reductive aldol reaction utilizing acetaldehyde is one of the key steps, together with a ruthenium-catalyzed alkene isomerization reaction.

Partial reduction of pyrroles: application to natural product synthesis

The partial reduction of N-Boc pyrroles has been explored giving stereoselective routes to disubstituted pyrrolines in good yields and with excellent diastereoselectivities. A novel methodology has been developed to carry out reductive aldol reactions on 2-substituted N-Boc pyrroles; use of aldehydes under reductive aldol conditions gave the anti aldol product in good selectivity. This chemistry was used as the key transformation in a synthesis of omuralide, which was achieved in 13 steps and 14% overall yield. We also report a methodology for selectively forming either cis or trans 2,5-disubstituted pyrrolines via a partial reduction of an electron-deficient N-Boc pyrrole. The trans pyrroline formed using this route was utilized in the syntheses of the polyhydroxylated pyrrolizidine natural products hyacinthacine A1 and 1-epiaustraline. Further investigation has led to the development of routes to enantiopure substituted pyrroline compounds. This has been achieved via a chiral protonation approach using easily accessible chiral acids, such as ephedrine and oxazolidinones, to quench enolates formed during the partial reduction process. Alternatively, enzymatic desymmetrization of symmetrical diol compounds formed from the partial reduction products of substituted pyrroles is also reported. This leads to formation of both enantiomers of 2,2- and 2,5-disubstituted N-Boc pyrrolines in excellent ee and yields.

Utility of the Ammonia-Free Birch Reduction of Electron-Deficient Pyrroles: Total Synthesis of the 20S Proteasome Inhibitor,clasto-Lactacystin β-Lactone

A new synthesis of the 20S proteasome inhibitor clasto-lactacystin beta-lactone is described. Our route to this important natural product involves the partial reduction of an electron deficient pyrrole as a key step. By judicious choice of enolate counterion, we were able to exert complete control over the stereoselectivity of the reduction/aldol reaction. Early attempts to complete the synthesis by using a C-4 methyl substituted pyrrole are described in full, together with our attempts to promote regioselective elimination of a tertiary alcohol. The lessons learnt from this first approach led us to develop another, and ultimately successful, route that introduced the C-4 methyl group at a late stage in the synthesis. Our successful route is then described and this contains several highly stereoselective steps including a cis-dihydroxylation and an enolate methylation. The final synthesis proceeds in just 13 steps and in 15 % overall yield making it an extremely efficient route to this valuable compound.