Cycloadditions - Carbohydrates: Tools for Stereoselective Synthesis - page 75

the diene substrate, all hetero-Diels-Alder reactions with d-manno compound 60
proceeded in fair to excellent yield and gave products 66, 68, 70, 72, 73 in excellent
ee (entries 1, 3, 5, 7, 9) [17]. The d-ribo derivative 61 proved to be a highly efficient
pseudo-enantiomer, giving products ent - 66, ent - 68, ent - 70, ent- 72, and ent- 73 in
almost identically good yields and ees (entries 2, 4, 6, 8, 10) [17]. Notably, in the
case of racemic substrate rac - 67 an efficient kinetic racemic resolution took place.
Unsymmetrically substituted substrate 6b gave rise to a regioisomeric mixture of
products 73a ent - 73b, respectively. While the regioselectivity

was modest, 73a and ent - 73a were obtained in high ee.
Reactions of meso- dienes 74 and 76 with d-manno heterodienophile 60
were reported by Werbitzky [18] and Piepersberg [19]. The corresponding
products 75 and 77 were formed in high yields and ees (Table 4.2, entries 11,
12) and were employed in the preparation of aminocyclitols 79 [18] and 81 [19]
(Scheme 4.12).

+ 73b and ent - 73a +

conduramine A-1

H N

NHAc NH 2

1.) Zn/HCl
2.) Ac 2 O, pyridi n e

Cl

O deacetylation

OAc OH

OAc

82%

OAc OH

OAc

OAc OH

75 78 79

H N

NH 2 NH 2

Cl

47% over 6 steps

O

HO OH

1.) Al/Hg, H 2 O, T HF O

CH 3
CH 3

O

94% O

H 2 N OH

H 3 C O

OH OH

81

CH 3

80

77

Scheme 4.12

Synthesis of aminocyclitols 79 and 81 from hetero-Diels-Alder products 74 and

76.

The group of Defoin used heterodienophile 60 in the preparation of some nojir-
imycin derivatives [20]. Scheme 4.13 shows the key steps towards 6-deoxy-l-
allo-nojirimycin ( 85 ) from diene 82 and heterodienophile 60 [20c]. Even though
82 was employed as a diastereomeric mixture of the (4 E/Z ) isomers, the cycload-
dition step gave 83 as the only stereoisomer. Probably, de-acetalization of 82 takes
place under the reaction conditions and the resulting aldehyde, which is prone to
(4 E/Z ) isomerization, reacts with 60. The resulting addition product is then
re-acetalized by methyl orthoformate [20c]. N-Protection and diastereoselective
dihydroxylation yielded 84 in 35% yield over three steps in excellent ee. This
product was subsequently transformed into 85.
As an alternative to 60, Wightman and coworkers designed -chloronitroso

compound 88, which is accessible from d-xylose ( 86 ) in 69% overall yield (Scheme
4.14) [21]. In 88 the chloronitroso moiety resides on C3 of the xylofuranose scaffold

α