Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0329 479

The reaction between diene 339 and 336 afforded cycloadduct 340 with 95% ee when catalyst 338 was used (equation 95). Adduct 340 was used as a precursor for the marine natural products gracillins B and C219.

SiMe3

H O t-Bu

N

H

O

(340)

2. Chiral boron catalysts

Chiral boron catalysts had already been widely used in a variety of reactions before they were applied in Diels –Alder reactions220. Boron catalysts were first employed in the Diels –Alder reactions of quinones with electron-rich dienes. Kelly and coworkers221 found that stoichiometric amounts of a catalyst prepared from BH3, acetic acid and 3,30 - diphenyl-1,10-bi-2-naphthol (344) catalyzed the reaction of 1-acetoxy-1,3-butadiene (341) with juglone (342) to afford cycloadduct 343 with 98% ee (equation 96). The reaction was supposed to proceed via a spirocyclic borate complex in which one face of the double bond of juglone was effectively shielded from attack by the diene.

A similar approach was followed by Yamamoto and colleagues222. A chiral boron catalyst prepared from trimethyl borate and various (R,R)-tartaric diamides 347 effectively catalyzed the cycloaddition of juglone to 1-triethylsilyloxy-1,3-butadiene (345) to give cycloadduct 346 with high enantioselectivity (equation 97).

The application of chiral boron catalysts in the cycloadditions of ˛,ˇ-unsaturated aldehydes and acrylic acid derivatives has been investigated most.

Kaufmann and colleagues examined the asymmetric induction of chiral boron complexes 348 and 349, obtained through reaction of HBBr2. SMe2 with pinene223 and 1,10 -

bi-2-naphthol224, respectively, in the cycloaddition of cyclopentadiene with methacrolein (equation 98). A low ee (28%) was found when employing catalyst 348, but the ee was greatly improved to 90% using catalyst 349. The X-ray structure of 349 showed the molecule to have a propeller-like shape with an interesting C3 symmetry.

Hawkins and Loren225 reported simple chiral arylalkyldichloroborane catalysts 352 which were effectively used in the cycloadditions of acrylates 11b and 350 to cyclopentadiene, affording adducts 351a and 351b, respectively (equation 99). A crystal structure of the molecular complex between methyl crotonate and the catalyst allowed the authors to rationalize the outcome of the reaction. One face of methyl crotonate is blocked by– donor–acceptor interactions, as becomes clear from the structure of complex 353. The cycloadduct of methyl acrylate and cyclopentadiene (5 equivalents) was obtained with 97% ee, using the same catalyst. Three years later, the authors reported that the cycloadduct was obtained with 99.5% ee in the presence of 10 equivalents of cyclopentadiene226.

O

O R

OMe

(353)

Kobayashi and colleagues227 prepared chiral boron reagent 355 from BBr3 and chiral prolinol derivative 354 (equation 100). This catalyst afforded the exo Diels –Alder adduct of cyclopentadiene and methacrolein with 97% ee (equation 101). In the same way, norbornene (2R)-357 was obtained from 356 and cyclopentadiene.

Chiral 1,2,3-oxazaborolidines simply obtained from ˛-amino acid derived sulfonamides and borane were first applied in Diels –Alder reactions by Taliasu and Yamamoto228, and Helmchen and colleagues229. Yamamoto prepared catalysts from ˛-aminobutyric acid derived arylsulfonamides and found that the enantioselectivity of the reaction between

The cycloaddition reaction of crotonaldehyde (362) with cyclopentadiene in the presence of 20 mo1% of catalyst 361b afforded cycloadduct 363 with 58% yield (endo/exo D 97/3) and 72% ee (equation 104)230.

O

Interestingly, Corey and coworkers231 showed that the main adduct in the reaction of methacrolein with cyclopentadiene was (2S)-322 (92% ee) when catalyst 364, derived from (˛S,ˇR)-ˇ-methyltryptophan and n-butylboric acid, was used.

NH

O

O N O

S B

O n-Bu

(364)

The effect of changing the position of the electron-donating atom in the side chain R of oxazaborolidine catalysts 367 was studied systematically for the reaction between cyclopentadiene and methacrolein. The enantioselectivity proved to be controlled by the presence of electron-donor atoms at positions 2 and 4 of the side chain. The effect was especially apparent in the formation of 366 from cyclopentadiene with ˛-bromoacrolein (365) (equation 105, Table 20), which is more electron-poor than methacrolein232.

These results were rationalized by application of a transition state model for the reaction catalyzed by 367d (Figure 6). A strong donor–acceptor interaction was envisaged between the oxygen atom of the benzyloxymethyl group and the carbonyl carbon of the complexed dienophile. In addition, a – stacking interaction between the aromatic ring and the olefinic double bond was proposed. Because of these interactions, one of the dienophile faces is selectively blocked for approach by cyclopentadiene.