Organic Syntheses, Coll. Vol. 7, p.470 (1990); Vol. 63, p.192 (1985).
A. Ricinelaidic acid
. Ricinoleic acid (Note 1) (39.75 g, 0.106 mol)
and 586 mg (2 mol %) of diphenyl disulfide
dissolved in 1000 mL of hexane
are placed in a photochemical reactor (Note 2)
and irradiated for 3 hr with a Philips HP(L) 250-W medium-pressure mercury lamp
. After irradiation the solvent is removed under reduced pressure and the semisolid residue is recrystallized from 185 mL of hexane
to yield 11.3 g
of crude ricinelaidic acid
, mp 39–43°C
. The irradiation is repeated with the mother liquor under the same conditions to yield, after removal of the solvent and recrystallization of the residue from 135 mL of hexane
, an additional 7.2 g
, mp 38–42°C
; total yield of crude ricinelaidic acid
is 18.5 g
). The product after recrystallization from 220 mL of hexane
weighs 15.6 g
), mp 43–45°C
, and is suitable for the following step. Repeated recrystallization from hexane
yields ricinelaidic acid
with mp 51.0–51.5°C (Note 3)
and (Note 4)
Technical-grade (80%) ricinoleic acid was obtained from Fluka AG Buchs, Switzerland or from Tridom Chemicals, Inc.
Saponification of methyl ricinoleate2
also gives suitable material.
The purity of the products has been checked by capillary gas liquid chromatography of the corresponding methyl ester obtained with ethereal diazomethane
solution (Carlo Erba Fractovap 20-m glass capillary coated with UCON HB at 160°C). Ricinelaidic acid
, mp 49–50°C
, contains 4%, that with mp 51.0–51.5°C
, less than 1% of ricinoleic acid
. Submitters obtained higher yields (58%
, mp 49–50°C
), perhaps due to better quality starting material.
obtained from Fluka AG, Buchs, Switzerland, was recrystallized from hexane (30 mL/g)
to yield a suitable product, mp 58–59°C
Silver perchlorate monohydrate (9 g)
(obtained from Fluka AG) is suspended in 110 mL of toluene
together with a Teflon-coated magnetic stirring bar
. The solution is magnetically stirred and heated in an oil bath until 70 mL of toluene
This reflux rate is crucial for predilution of the carbothioate in the condenser. Lower reflux rates require an accordingly slower addition of the S
-(2-pyridyl)carbothioate during 2–4 hr.
Silica gel 60 Merck in a 2.5-cm-diameter column was used.
The product distills at 110°C (0.01 mm) in a Kugelrohr distillation apparatus
and has an optical rotation of [α]D +42°
The silver ion-promoted lactonization of hydroxy-S
-(2-pyridyl)carbothioates was introduced by the submitters3
as a mild method for the synthesis of naturally occurring macrolides as, for example, nonactin4
from the corresponding hydroxy acids. If the method of Mukaiyama et al.6
is used for the formation of the S
-(2-pyridyl)carbothioate, no protection of the hydroxyl group is needed in this step. The cited examples show that silver ion-promoted lactonization can be used to effect ring closure of base-sensitive and unsaturated acid-sensitive hydroxy acids in good yield.
Similar methods to effect lactonization have been proposed by Corey et al.7
and Masamune et al.8
The first consists of prolonged heating of hydroxy-S
-(2-pyridyl)carbothioates in boiling xylene
; the second is the mercury trifluoroacetate-promoted cyclization of a hydroxy-S-tert-butyl carbothioate
was selected for the submitted procedure because it has a moderately complex structure and can be prepared easily from commercially available technical grade ricinoleic acid
. This conversion represents an example of the facile cis–trans interconversion of olefins9
caused by photochemically generated phenylthiyl radicals leading to the thermodynamic equilibrium.
This preparation is referenced from:
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