Complex sentence: Details of Rosenberg's life before 1880 are mostly unknown, however, while living in San Francisco in the 1880s, he developed a reputation as an eccentric among the local Jewish community.
Split into simpler sentences: Details of Rosenberg's life before 1880 are mostly unknown. However, he drew attention while he lived in San Francisco in the 1880s and developed a reputation as an eccentric among the local Jewish community.