Organic Syntheses, CV 6, 670
NITRONES FOR INTRAMOLECULAR 1,3-DIPOLAR CYCLOADDITIONS: HEXAHYDRO-1,3,3,6-TETRAMETHYL-2,1-BENZISOXAZOLINE
[2,1-Benzisoxazole, 1,3,3a,4,5,6,7,7a-octahydro-1,3,3,6-tetramethyl-]
Submitted by Norman A. LeBel and Dorothy Hwang
1.
Checked by Christopher K. VanCantfort and Robert M. Coates.
1. Procedure
A
1-l., three-necked, round-bottomed flask is fitted with a
mechanical stirrer, a
reflux condenser attached to a
Dean-Stark water separator, and a
250-ml. dropping funnel. The flask is charged with
25.0 g. (0.16 mole) of 3,7-dimethyl-6-octenal (Note
1) and
500 ml. of toluene. The solution is heated to reflux with stirring, and a solution of
N-methylhydroxylamine,
methanol, and
toluene is added (see below).
To a cooled and magnetically stirred solution of
23.4 g. (0.280 mole) of N-methylhydroxylamine hydrochloride (Note
2) and (Note
3) in
40 ml. of methanol is added
15.3 g. (0.282 mole) of sodium methoxide. The cooling bath is removed, and the mixture is stirred at room temperature for 15 minutes. The mixture is filtered rapidly through a
35-mm., coarse, sintered-glass funnel, and the filter cake is washed with
10 ml. of methanol. The filtrates are combined, refiltered, and mixed with
150 ml. of toluene.
The two-phase mixture containing
N-methylhydroxylamine is added dropwise to the refluxing
toluene solution of the aldehyde over 3 hours. During this time the distillate is collected and discarded in 25-ml. portions until the last such portion collected and discarded is clear (Note
4). Reflux and stirring are continued for an additional 3 hours, and then the clear reaction mixture is allowed to cool. The product is extracted with three
80-ml. portions of 10% hydrochloric acid. The extracts are combined, and the pH of the solution is adjusted to >12 by the slow addition of
30% aqueous potassium hydroxide. The basic mixture is extracted with two
120-ml. portions of pentane (Note
5), and the combined extracts are washed once with 100 ml. of water and dried over anhydrous
potassium carbonate. The
pentane is removed on a
rotary evaporator, and the residue is distilled through a short
Vigreux column under reduced pressure, yielding
19.1–19.6 g. (
64–67%) (Note
6) of
hexahydro-1,3,3,6-tetramethyl-2,1-benzisoxazoline, b.p.
90–92° (9 mm.) (Note
7).
2. Notes
2. This quantity is a 0.72 molar excess. A molar excess of at least 0.5 is needed to maximize the yield.
4. Water and
methanol are removed by this procedure, so that a higher reaction temperature can be achieved.
5. One to four additional extractions improve the yield slightly.
7. A GC analysis carried out by the submitters using a
1.85 m. × 0.32 cm. stainless-steel column packed with
10% Polyglycol E-20M on Chromosorb W at 150° indicated that the product is a mixture of
trans,trans and
cis,trans stereoisomers in a ratio of 89 : 11. The spectral properties of the product are: IR (thin film) cm
−1: 1462, 1387, 1348, 1287, 1277, 1193, 1179, 1136, 1124, 935, 915, 877, 810;
1H NMR (CDCl
3), δ (multiplicity, coupling constant
J in Hz., number of protons, assignment): 1.00 (d,
J = 7, 3H, C
H3), 1.08 (s, 3H, C
H3), 1.28 (s, 3H, C
H3), 2.60 (s, 3H, NC
H3).
3. Discussion
This procedure is an adaptation
3 of the original method,
4 and avoids the isolation and purification of
N-methylhydroxylamine. Nitrones undergo 1,3-dipolar cycloadditions with a wide variety of dipolarophiles (see recent review
5). The intramolecular variation represents a useful synthetic approach, as carbocyclic or heterocyclic rings are generated together with the five-membered isoxazolidines.
6 The intermediate nitrone is usually not preformed (present example), although intermolecular cycloaddition is rarely a problem.
N-alkyl-,
N-alkenyl-, and
N-arylhydroxylamines have been used with aldehydes and ketones to generate the nitrones
in situ, and some typical examples are listed in Table I. Cyclic azomethine imine oxides are also important substrates for intramolecular cycloadditions.
6
TABLE I
INTRAMOLECULAR 1,3-DIPOLAR CYCLOADDITIONS OF NITRONES TO ALKENES
|
Carbonyl Compound |
Hydroxylamine |
Product |
Yield (%) |
|
CH2=CH(CH2)3CHO |
CH3NHOH |
|
414 |
|
C2H5NHOH |
|
774 |
|
CH3NHOH |
|
8715 |
|
CH3NHOH |
|
7815 |
|
CH3NHOH |
|
807 |
|
(CH3)2CHNHOH |
|
70–808 |
|
C6H5NHOH |
|
889 |
|
CH3NHOH |
|
55.410 |
|
CH3NHOH |
|
8011 |
|
CH3NHOH |
|
1012 |
|
Although isoxazolidines are less basic than the analogous amines,
N-alkylisoxazolidines can form quaternary ammonium salts. Reductive cleavage of isoxazolidines and the methiodides can be effected with various reagents (
zinc–
acetic acid,
hydrogen–
palladium,
lithium aluminum hydride), and the yields of 1,3-aminoalcohols are generally excellent.
4 Other reagents that result in modifications of the isoxazolidine ring include peroxyacids,
13 strong bases,
14 triplet photosensitizers,
14 and
cyanogen bromide.
15
Appendix
Compounds Referenced (Chemical Abstracts Registry Number)
Hexahydro-1,3,3,6-tetramethyl-2,1-benzisoxazoline
2,1-Benzisoxazole, 1,3,3a,4,5,6,7,7a-octahydro-1,3,3,6-tetramethyl-
potassium carbonate (584-08-7)
hydrochloric acid (7647-01-0)
acetic acid (64-19-7)
methanol (67-56-1)
ammonium chloride (12125-02-9)
hydrogen (1333-74-0)
sodium methoxide (124-41-4)
potassium hydroxide (1310-58-3)
toluene (108-88-3)
zinc (7440-66-6)
palladium (7440-05-3)
Pentane (109-66-0)
hydroxylamine (7803-49-8)
Nitromethane (75-52-5)
Cyanogen bromide (506-68-3)
lithium aluminum hydride (16853-85-3)
3,7-dimethyl-6-octenal,
citronellal (106-23-0)
3,7-dimethyl-7-octenal
N-methylhydroxylamine (593-77-1)
N-methylhydroxylamine hydrochloride (4229-44-1)
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