Polyene cyclization enantioselectivity

Enantioselective Polyene Cyclization Catalyzed SnCU-BINOL Derivatives. Non-enzymatic enantioselective polyene cyclizations are very attractive alternatives to the multistep synthesis from naturally occurring chiral synthons. The authors have succeeded in the first enantioselective biomimetic cyclization of polyprenoids catalyzed by LBA. (—)-Ambrox is the most important commercial substitute for ambergris, due to its unique olfactory and fixative properties. The successful preparation of (—)-ambrox has been achieved by the enantioselective cyclization of homofamesol promoted by (/ )-BINOL-Me-SnCl4, although the enantioselectivity and diastereoselectivity is moderate (eq 10). 

Enantioselective Polyene Cyclization Catalyzed by SnCl4 BINOL Derivatives... 

Non-enzymatic enantioselective polyene cyclizations are very attractive alternatives to multistep synthesis from naturally occurring chiral synthons. 

Another example of an enantioselective polyene cyclization is the formation of tetracycle 103 (Scheme 12.23). Enantioselectivities of the 6-exo followed by 6-endo cyclization were moderate to good using BINAP. Of note is the substantial increase in enantioselectivity... 

SCHEME 9.13 Enantioselective polyene cyclization promoted by LBA 7-SnCl. ... 

In 2007, Gagne s group succeeded in a regio- and diastereoselective oxidative polycyclization of di- and trienols catalyzed achiral [(dppe)Pt] dications, wherein turnover was achieved by the trityl cation abstracting a hydride from a putative [(dppe)Pt-H] intermediate [32i]. One year later, Gagne s group developed the catalytic enantioselective polyene cyclization induced by [(S)-(xylyl-PHANEPHOS) Pt][(BF )j] catalyst, which was prepared from (S)-(xylyl-PHANEPHOS)PtIj and AgBF in situ (Scheme 9.21) [32j]. This asymmetric catalysis enables the oxidative cascade cyclization of polyalkene substrates [32k] (Scheme 9.15). 

In 2013, Gagne s group demonstrated catalytic enantioselective polyene cyclization and C3-fluorination for... 

SCHEME 9.19 Enantioselective polyene cyclizations promoted by dicationic Pt(II) complexes. 

SCHEME 9.21 Chiral Pt(ll)-induced enantioselective polyene cyclizations. 

In 2010, Toste s group reported the first example of a highly enantioselective polyene cyclization reaction in which transition metal-promoted alkyne activation serves as the cyclization initiating event [34], The (MeO-DTBM-BIPHEP)gold(I)-catalyzed reaction offers an efficient method for the stereoselective synthesis of polycyclic compounds... 

In 2010, Loh s group reported a highly efficient catalytic enantioselective polyene cyclization [52]. An a-ketoester was demonstrated to initiate enantioselective cationic polyene cyclization catalyzed by a Sc(OTf)3-Pybox Lewis acid catalyst (Scheme 9.39). 

These catalytic enantioselective polyene cyclizations have been accomplished through the use of substrates with the appropriate functional groups on the reactive sites, such as aldehydes and hydroxylactams. Therefore, these methods cannot be applied to the enantioselective cyclization of simple isoprenoids with a terminal isoprenyl group. 

SCHEME 9.40 Enantioselective polyene cyclization via organo-SOMO catalysis. 

In 2014, Corey s group demonstrated the synthetic application of LBA (/ )-13 Sba5 in catalyzed enantioselective polyene cyclization [21b] by three different ways in which the LBA could be used to initiate enantioselective, cationic polycycUzations at a nonterminal jt-bond. 

The first way involves activation of an internal double bond by an electron-supplying jt-attachment [21b]. This type of process is quite useful, as is demonstrated by the especially simple, enantioselective syntheses of three well-known natural products. Thus, (-i-)-dysideapalaunic acid, dehydroabietic acid, and 4-epipodocarpic acid were synthesized through very short pathways including an enantioselective polyene cyclization under the influence of the LBA at -78°C for 15 min (Scheme 9.50). 

Chiral pyrogallol (14) derived LBA (15) was developed for enantioselective polyene cyclization reaction as an artificial cyclase by Yamamoto, Ishihara and coworkers in 2004 (Scheme 1.23) [26]. Various tricyclic skeletons have been synthesized with good enantioselectivities (79-85% ee) in the presence of chiral LBA (15). 

Scheme 1.23 SnCU-chiral pyrogallol complex (15) as a new LBA for enantioselective polyene cyclization.

Yamamoto and Ishihara et al. developed enantioselective polyene cyclization with predictable absolute configurations and excellent enantioselectivities using the chiral LBA (Scheme 10.23) [45]. They further succeeded in produdng new chiral LBA consisting of chiral pyrogallol derivatives and SnCU, which effedively promotes the cyclization of 2-geranylphenol derivatives (Scheme 10.24) [46]. 

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