No, it is not possible to force the system to sync with your data base connected via LDAP.
The system is configured to connect refresh every 3 hours and transfer any new information added into the directory service to the Control Panel. This intervall was chosen in order to allow you some time to identify and retract any potentially harmful mis-configuration done on the directory service side.
Example: Let's assume the worst case scenario, and your Active Directory lost all stored information. You luckily have a backup that you can use. But unless you have foresight, there will be a delay between discovering the data loss and being able to revert to the backup.
If the LDAP synchronization would trigger instandly, you would also lose and users stored in the Control Panel in addition to the already lost LDAP users. Depending on the user check setup for Spam and Malware Protection this might also result in all incoming emails being rejected. The sync taking place later on, will allow you some leeway in response time to the initial issue.