Reaction of o-phthalaldehyde and thiols with primary amines: formation of 1-alkyl(and aryl)thio-2-alkylisoindoles

Abstract

The fluorogenic reaction of o-phthalaldehyde (OPTA) and /3-mercaptoethanol (MERC) with primary amino acids gives a 1-alkylthio-2-alkylisoindole as the product. The structure of this group of previously unknown isoindoles was determined (1) from an in situ analysia of the adducts formed in solution from OPTA, MERC or ethanethiol (ET), and n-propylamine, (2) from the characterization of solid derivatives of these MERC and ET adducts, and (3) from the studies of two isolable isoindoles (an OPTAltert-butylthiol/n-propylamine adduct and a dimeric adduct formed from OPTA, ethanedithiol, and ri-propylamine). The reaction is found to be quite general as OPTA and numerous thiols rapidly react with n-propylamine or leucine to give isoindoles in excellent yield. Most adducts were not isolated, but their physical properties in solution were qualitatively identical with those of the tert-butyl and dimeric ethanedithiol adducts. Analyses of the chemical shifts of the isoindole alkyl substituents in the NMR spectra support the previous conclusions that these heterocycles have relatively low levels of aromatic character. The addition sequence of the reaction is important; the best results are obtained by mixing OPTA and thiol before adding the amine. With some thiols, this procedure results in the initial formation of a l-alkylthio-3-hydroxy-1,3-dihydroisobenzofuran. However, these 1:l adducts do not appear to be obligatory intermediates in the subsequent reaction with primary amines to form 1-thio-sulistituted isoindoles.The fluorogenic reaction of o -phthalaldehyde (OPTA) and 6-mercaptoethanol (MERC) with amineslJ and amino acids and protein^^-^ has recently attracted much attention due to the high sensitivity of the assay which can be conducted in aqueous solutions. Thus, picomole quantities of amino acids can be readily detected. In recent preliminary communications we deduced that these intensely fluorescent OPTA reaction products are 1-alkylthio-2-alkyl-substituted isoindoles 1.2.10 d-R' N-R' 1 R ' = alkyl or aryl; RZ = alkyl Isoindoles in general are quite reactive and eluded isolation until 1951.11812 In spite of their 10-a-electron apparently aromatic structure, N-substitution and especially halogen s u b~t i t u t i o n l~-'~ are required to increase the stability of isoindoles not conjugated with other unsaturated functional groups. Very few 0xygen-~3%1~ or nitrogen-substituted17 isoindoles have been reported, and the effect of such subsdtution on isoindole stability is not yet clear. For this reason, thiosubstituted isoindoles l are an important new class of hetero-substituted isoindoles which should be useful in further defining the physical and chemical properties of these interesting 10-a-electron bicyclic heterocycles. In this paper we present the details of our preliminary communicati o n~,~J~ describe the preparation and physical properties of several new 1-alkyl(and aryl)thio-2-alkylisoindoles, and assess the scope and mechanism of the OPTA reaction. A discussion of the fluorescence properties of these compoun...

Thiosugars – Derivatization agents for chiral resolution of homoleucines

“…The reaction in borate was complete within one minute at ambient temperature. In the course of reaction, amino acids bearing primary amino groups form highly fluorescent derivatives which were assigned, by analogy with the reaction with mercaptoethanol or glutathione [26,27], to 1-isoindolyl-(1-thioglycosides) (Fig. 1).…”

Chemical Modification of Potato Phosphorylase byo-Phthalaldehyde

“…Based on the molar absorption coefficient of the isoindole ring at 337nm (7.66mM-1 cm-I ), 8) it was confirmed that about 1.25mol of OPA were incorporated into I mol of potato phosphorylase monomer. The increase in the UV absorption at 337 nm reached a maximum after 120 min at 30°C (Fig.…”

Update and Evaluation of the Effectiveness of Different Thiols and Micellar Media in Roth's Fluorimetric Method for the Determination of Primary Amino Compounds

“…The decomposition of 1-alkylthio-2-alkylisoindoles formed in buffered aqueous media have been postulated to be due to autoxidation (78,129) and/or nonoxidative nucleophilic or solvolytic reactions (160) among others (4,6,89). In addition, OPA itself is known to undergo a variety of chemical and photochemical reactions (161)(162)(163)(164)(165)(166).…”