Isoprene Lewis acid promoted

Diels-Alder Cycloadditions (AUcene -+ Six-Membered Cycloadduct). lV-Acryloyl-a-methyltoluene-2,a-sultam (3a) participates in highly endo and C(a)-re ir-face selective Lewis acid promoted Diels-Alder reactions with Cyclopentadiene, 1,3-Butadiene, and Isoprene (eq 2 and Table 1). These levels of induction compare favorably with most alternative auxiliaries, including the 10,2-camphorsultam. However, V-crotonyl-a-methyltoluene-2,a-sultam (ent-3b) reacts with cyclopentadiene with only mod-... 

An extremely useful enantioselective Diels-Alder reaction which uses a super-reactive cationic oxazaborinane catalyst was recently reported by Corey and co-workers (Eq. 25) [25]. This strong chiral Lewis acid promotes the Diels-Alder reaction between reactive and unreactive dienes and dienophiles. With tetrabromoborate as a counter-ion, good enantioselectivities were achieved in the reaction of cyclopenta-diene with several a,/3-enals. With tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as a counter-ion, the reaction of isoprene and a-bromoacrolein at -94 °C gave the desired cycloadduct in 90 % yield and 96 % ee. 

To overcome these problems with the first generation Brmsted acid-assisted chiral Lewis acid 7, Yamamoto and coworkers developed in 1996 a second-generation catalyst 8 containing the 3,5-bis-(trifluoromethyl)phenylboronic acid moiety [10b,d] (Scheme 1.15, 1.16, Table 1.4, 1.5). The catalyst was prepared from a chiral triol containing a chiral binaphthol moiety and 3,5-bis-(trifluoromethyl)phenylboronic acid, with removal of water. This is a practical Diels-Alder catalyst, effective in catalyzing the reaction not only of a-substituted a,/ -unsaturated aldehydes, but also of a-unsubstituted a,/ -unsaturated aldehydes. In each reaction, the adducts were formed in high yields and with excellent enantioselectivity. It also promotes the reaction with less reactive dienophiles such as crotonaldehyde. Less reactive dienes such as isoprene and cyclohexadiene can, moreover, also be successfully employed in reactions with bromoacrolein, methacrolein, and acrolein dienophiles. The chiral ligand was readily recovered (>90%). 

Cycloalkenones generally perform poorly as dienophiles in Diels-Alder reactions but their reactivity can be enhanced by Lewis acids [105]. SnCU is effective in promoting the Diels-Alder reaction between simple 1,3-butadienes, for example isoprene and piperylene, and cyclopentenone esters (Eq. 67) [106], Cycloaddition does not, however, occur in the presence of SnCU when the diene contains an oxygen-bearing substituent such as an alkoxy or siloxy group. For such compounds, as is generally true for the Diels-Alder reactions of cycloalkenones, other Lewis acids such as zinc chloride are more effective. 

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