A Highly Stereoselective Approach to the Synthesis of Functionalized Pyran Derivatives by Lewis Acid Assisted Ketal Reduction and Allylation

Abstract

Reduction of bicyclic ketal 1 gave functionalized pyran derivatives 7a or 7b in a highly stereoselective manner, depending upon the reduction conditions utilized. For example, treatment of ketal 1 with TiCl4/Et3SiH produced exclusively diol 7b with the 2,5-syn relationship in good yield. Alternatively, reduction of ketal 1 by DIBALH gave 2,5-anti-diol 7a stereoselectively. Alane reductions of ketal 1 were highly stereoselective also; however, the syn/anti selectivity observed was strongly dependent on the ratio of reagents employed for in situ generation of the alane. Lewis acid catalyzed allylation of ketal 1 gave pyran 10 in a stereospecific alkylation reaction.

Diastereoselective Aldol Reactions of Furaldehyde Using a Chiral Boronate as Auxiliary: Application to the Synthesis of Enantiomerically Pure and Highly Functionalized 2,3-Disubstituted Furanyl Alcohols

“…To access these compounds through an oxidative rearrangement of furanyl alcohols, a variety of reagents have been reported. For example, m ‐chloroperbenzoic acid ( m CPBA),3 peracetic acid,4 pyridinium chlorochromate (PCC),5 iodobenzenediacetate (IBDA)6 and N ‐bromosuccinimide (NBS)7 were all employed as oxidants. The use of bromine in methanol,8 bromine in water9 and anodic oxidation10 has also been reported.…”

Chemistry of Tonghaosu Analogs: Novel Acid‐Catalyzed Nucleophilic Addition to the Dienyl Acetal System

“…[11] In the presence of catalytic amounts of AlCl 3 , compound 2c was reduced with lithium aluminum hydride (LiAlH 4 ) to yield a 1:1 mixture of furan 7 and dihydrofuran 8 (Scheme 1). …”

“…The reduction of ketals is a useful approach for the stereoselective synthesis of cyclic ethers, and diisobutylaluminum hydride or a combination of a Lewis acid and a reducing agent is often employed for this purpose. [11] In the presence of catalytic amounts of AlCl 3 , compound 2c was reduced with lithium aluminum hydride (LiAlH 4 ) to yield a 1:1 mixture of furan 7 and dihydrofuran 8 (Scheme 1). Scheme 1.…”

Diastereoselective Aldol Reactions of Furaldehyde Using a Chiral Boronate as Auxiliary: Application to the Synthesis of Enantiomerically Pure and Highly Functionalized 2,3‐Disubstituted Furanyl Alcohols.

“…4 and halide in the solvent mentioned, under nitrogen at room temp. The mixture was then stirred for 2 h at 0°C; Et 2 O (60 mL) was then added to the reaction mixture.…”

“…[1] Many methods have therefore been developed in order to realize these compounds. For example, m-chloroperbenzoic acid (mCPBA), [3] peracetic acid, [4] pyridinium chlorochromate (PCC), [5] iodobenzenediacetate (IBDA) [6] and N-bromosuccinimide (NBS) [7] were all employed as oxidants. For example, m-chloroperbenzoic acid (mCPBA), [3] peracetic acid, [4] pyridinium chlorochromate (PCC), [5] iodobenzenediacetate (IBDA) [6] and N-bromosuccinimide (NBS) [7] were all employed as oxidants.…”