Carbocyclization Lewis acid catalyzed

Mascarenas developed a synthetic method to 1,5-oxygen-bridged medium-sized carbocycles through a sequential ruthenium-catalyzed alkyne-alkene coupling and a Lewis-acid-catalyzed Prins-type reaction (Eq. 3.45). The ruthenium-catalyzed reaction can be carried out in aqueous media (DMF/H20 = 10 1).181... 

Finally in 2005, Hutt and Mander reported their strategy for the synthesis of nominine (Scheme 1.3) [29], The approach relies upon construction of the steroidal ABC carbocyclic ring structure followed by stepwise preparation of the fused aza-ring system. In the key sequence of the synthetic study, enone 50 was oxidized to dienone 51 with DDQ followed by Lewis acid-catalyzed intramolecular conjugate addition of the methylcarbamate to the newly formed dienone to deliver pyrrolidine 52. 

Diels-Alder reactions with acyclic and carbocyclic dienes are compiled in Scheme 5. The comparison between the Lewis-acid catalyzed and pressure-induced reaction (entry 1) shows that the application of high pressure, particularly in acid-sensitive systems, can sometimes lead to a better yield. Furthermore, pressure may shift the product ratio, if the activation volumes of the competing reactions are different, so that the application of pressure may also be useful in highly reactive systems, e.g. the reactive indenone 17 as dienophile, provided that a shift in the product ratio is desired. At atmospheric... 

Lewis acid catalyzed intramolecular coupling of allylsilanes with aziridines serves as a useful route for the synthesis of carbocycles having the y-amino olefin unit 104 (equation 80)149. However, this reaction does not occur in an intermolecular manner. 

Lewis acid catalyzed intramolecular alkylations of silyl enol ethers containing 5n1-reactive functionality provide useful routes to a variety of carbocyclic systems. - Smith et al have employed an intramolecular Mukaiyama reaction of the enol derivative (76) to produce the tetracyclic system (77) (equation 7). This transformation was a key step in their elegant synthesis of jatrophone. The synthesis... 

Early applications of chiral Lewis acid catalyzed stereoselective Diels Alder reactions used either boron- or aluminum-derived systems in carbocyclic ring formation18 1Q, or studied the effect of chiral shift reagents, such as Eu(hfc)3, in hetero-Diels-Alder cycloadditions of carbonyl compounds to dienes20 23,77, 78. The latter type of transition metal catalyzed addition is classified as heterocarboration and is described in Section 1.5.8.4. 

The important feature of the Lewis acid-catalyzed rearrangement is that these reactions are applicable to C-glycosides tScheme 12.10. 30 31) when the substrates possess electron-donating aglycons. S- and Se-Glycosides also afforded the corresponding carbocycles (32 33)P... 

The anions, generated in situ by desilylation of silylacetylenes, allylsilanes, propargylsilanes, a-silyloxetanones, bis(trimethylsilylmethyl) sulfides, and other silane derivatives,can undergo nucleophilic addition to ketones and aldehydes (eq 11). Al-(C,C-bis(trimethylsilyl)methyl) amido derivatives can add to aldehydes followed by Peterson alkenation to form acyl enamines. Treatment of 2-trimethylsilyl-l,3-dithianes can generate dithianyl anions, which are capable of carbocyclization via direct addition to carbonyl or Michael addition (eq 12). The fluoride-catalyzed Michael additions are more general than Lewis acid-catalyzed reactions and proceed well even for those compounds with enolizable protons and/or severe steric hindrance (eq 13). ... 

Keck and co-workers have investigated the exo cyclizations of the allylstan-nanes (Z)-158 and ( )-159 these reactions can be promoted by both thermal and Lewis acidic conditions (Table 11-10) [54]. As illustrated in Table 11-10, the (Z)-stannane 158 preferentially forms the 1,2-syn adduct 160 under both thermal promotion and catalysis by Bp3 OEt2. In contrast, the (i )-stannane 159 cyclizes to form the, 2-anti carbocycle 161 as the major adduct in the BF -OEta-catalyzed allylation, while the, 2-syn carbocycle 160 is the major adduct in the thennji] allylation process. 

Direct Wittig reaction of Ph PCHCOn,Me with the four unsubstituted D-aldopentoses followed by acetylation provides convenient preparative access to acyclic seven-carbon trans-2.3-unsaturated sugar derivatives. These products served as dienophiles for a detailed comparative study in Diels—Alder cycloaddition with cyclopentadiene. Related syntheses afforded analogous cis-dienophiles. Cycloaddition under uncatalyzed thermal conditions gave mixtures of the four possible stereoisomeric norbornene adducts. The endo, exo ratios, and diastereofacial selectivities of the adducts were determined by NMR spectroscopy and by chemical transformations, supplemented by selected X-ray crystallographic analyses. Different distributions of isomers were encountered when a Lewis acid was used to catalyze the cycloaddition. The reaction can be controlled to provide preparative access to selected isomers and thus constitutes a versatile method for chirality transfer from the precursor sugar to four new asymmetric centers in a carbocyclic framework. 

Reductive Carbocyclization of Dienes. Carbocyclization-silylation of 1,5- and 1,6-dienes such as 39 (52-58) as well as enynes (59) catalyzed by Cp 2YMe THF or Cp 2LuMe THF is a highly regio- and diastereoselective process, which tolerates various functional groups such as ethers, thioacetals, and tertiary amines in spite of the strong Lewis acidity of the catalysts (Scheme 22). This reaction has been applied to the synthesis of izidine alkaloids such as epilupinine 43 (Scheme 23) (54). 

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