Azithromycin, 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A (N-methyl-11-aza-10-deoxo-10-dehydroerythromycin A: IUPAC) of formula (II) disclosed in U.S. Pat. Nos. 4,517,358 and 4,474,768, is an azalide-type semi-synthetic macrolide antibiotic, useful for treating bronchial infection, sexual contact infection and dermatological infection (See Kirste and Sides; Antimicrob. Agents Chemother., 33, 1419(1989)).

Azithromycin is known to exist in three forms, the anhydride, monohydrate and dihydrate forms. These forms have been identified by powder X-ray diffraction and differential scanning calorimetric studies.
Azithromycin anhydride, which is disclosed in U.S. Pat. No. 4,517,359, is non-crystalline product and thus, its highly hygroscopic property is not suitable for pharmaceutical formulation.
Further, azithromycin monohydrate (mp. 136° C.), as described in U.S. Pat. No. 4,474,768 and WO Publication No. 89/00576, is crystalline but it has also hygroscopic property, making it difficult to maintain its water content at a constant level.
WO Publication No. 89/00576 discloses a process for preparing azithromycin dihydrate (mp. 126° C.) from azithromycin monohydrate by recrystallizing from a mixture of tetrahydrofuran, water and a C5˜C7 aliphatic hydrocarbon. Although the dihydrate is less hydroscopic than the monohydrate, the water content thereof must be carefully maintained during a vaccum drying step at a relatively low temperature. Such a water content controlling procedure is, however, not sufficient for removing the toxic aliphatic hydrocarbon solvent rigorously used in the recrystallization procedure. On the other hand, vacuum drying in higher temperature may result in formation of azithromycin dihydrate having undesirable water content.
Accordingly, many attempts have been made to develop a novel crystal or solvate form of azithromycin. For example, EP Publication No. 0,984,020 discloses a clathrate of azithromycin monohydrate with isopropanol of formula (III).

WO Publication No. 00/32203 discloses an ethanol solvate of azithromycin hydrate of formula (IV).

However, there has existed a need to develop an improved crystal form of azithromycin crystal suitable for pharmaceutical applications.